Semiconductor device

ABSTRACT

A semiconductor device that is less influenced by variations in characteristics between transistors or variations in a load, and is efficient even for normally-on transistors is provided. The semiconductor device includes at least a transistor, two wirings, three switches, and two capacitors. A first switch controls conduction between a first wiring and each of a first electrode of a first capacitor and a first electrode of a second capacitor. A second electrode of the first capacitor is connected to a gate of the transistor. A second switch controls conduction between the gate and a second wiring. A second electrode of the second capacitor is connected to one of a source and a drain of the transistor. A third switch controls conduction between the one of the source and the drain and each of the first electrode of the first capacitor and the first electrode of the second capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/034,440, filed Jul. 13, 2018, now allowed, which is a continuation ofU.S. application Ser. No. 15/059,549, filed Mar. 3, 2016, now U.S. Pat.No. 10,056,413, which is a continuation of U.S. application Ser. No.13/650,939, filed Oct. 12, 2012, now U.S. Pat. No. 9,280,931, whichclaims the benefit of foreign priority applications filed in Japan asSerial No. 2011-288418 on Oct. 18, 2011, and Serial No. 2011-261317 onNov. 30, 2011, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device, alight-emitting device, or a display device. Alternatively, the presentinvention relates to a method for driving or manufacturing the device.An example of the semiconductor device is a semiconductor deviceincluding, for example, an active element such as a transistor. Anexample of the light-emitting device is a light-emitting deviceincluding a light-emitting element such as an electroluminescent element(hereinafter also called EL element). An example of the display deviceis a display device including a light-emitting element, such as an ELelement, or a display element. The present invention relatesparticularly to the semiconductor device, the light-emitting device, orthe display device in which the influence of variations incharacteristics between transistors is reduced, or to a method fordriving the device.

BACKGROUND ART

Since display devices using light-emitting elements have highvisibility, are suitable for reduction in thickness, and do not havelimitations on viewing angles, they have attracted attention as displaydevices which can take the place of CRTs (cathode ray tubes) or liquidcrystal display devices. Specifically proposed structures of activematrix display devices using light-emitting elements are differentdepending on manufacturers. However, in general, at least a lightemitting element, a transistor (a switching transistor) which controlsinput of video signals to pixels, and a transistor (a drivingtransistor) which controls value of current supplied to thelight-emitting elements are provided for each pixel.

For example, when all the transistors in pixels have the sameconductivity type, it is possible to omit some of steps for fabricatingthe transistors, for example, a step of adding an impurity elementimparting one conductivity type to a semiconductor film. Patent Document1 discloses a display device in which transistors included in pixels areall n-channel transistors.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2003-195810

DISCLOSURE OF INVENTION

In a semiconductor device such as a light-emitting device or a displaydevice, drain current of a transistor is supplied to a light-emittingelement; thus, when characteristics and the like of transistors varyamong pixels, the luminance of display elements such as light-emittingelements varies correspondingly. Thus, in order to improve the qualityof a semiconductor device, it is important to propose a pixel structurein which the amount of drain current of a transistor can be corrected inanticipation of variations in threshold voltage, for example

In view of the above problem, it is an object of one aspect of thepresent invention to provide a semiconductor device, a light-emittingdevice, or a display device that is less influenced by variations incharacteristics of transistors. It is an object of one aspect of thepresent invention to provide a semiconductor device, a light-emittingdevice, or a display device that is less influenced by degradation ofcharacteristics of a transistor. It is an object of one aspect of thepresent invention to provide a semiconductor device, a light-emittingdevice, or a display device in which variations in luminance due tovariations in threshold voltage of transistors are reduced. It is anobject of one aspect of the present invention to provide a semiconductordevice, a light-emitting device, or a display device in which variationsin luminance due to variations in mobility of transistors are reduced.It is an object of one aspect of the present invention to provide asemiconductor device, a light-emitting device, or a display device thatcorrectly operates even when using a normally-off transistor. It is anobject of one aspect of the present invention to provide a semiconductordevice, a light-emitting device, or a display device in which thethreshold voltage of a transistor can be obtained even when thetransistor is a normally-off transistor. It is an object of one aspectof the present invention to provide a semiconductor device, alight-emitting device, or a display device that displays high-qualityimages. It is an object of one aspect of the present invention toprovide a semiconductor device, a light-emitting device, or a displaydevice that displays images with little unevenness. It is an object ofone aspect of the present invention to provide a semiconductor device, alight-emitting device, or a display device in which a desired circuitcan be formed with a small number of transistors. It is an object of oneaspect of the present invention to provide a semiconductor device, alight-emitting device, or a display device in which a desired circuitcan be formed with a small number of wirings. It is an object of oneaspect of the present invention to provide a semiconductor device, alight-emitting device, or a display device that is less influenced bydegradation of a light-emitting element. It is an object of one aspectof the present invention to provide a semiconductor device, alight-emitting device, or a display device that is manufactured in asmall number of steps.

Note that the description of these objects does not impede the existenceof other objects. In one aspect of the present invention, there is noneed to achieve all the objects. Other objects will be apparent and canbe derived from the description of the specification, the drawings, theclaims, and the like.

A semiconductor device according to one aspect of the present inventionincludes at least a transistor, a first wiring, a second wiring, a firstswitch, a second switch, a third switch, a first capacitor, and a secondcapacitor. The first switch has a function of selecting conduction ornon-conduction between the first wiring and one electrode of a pair ofelectrodes of the first capacitor. The one electrode of the pair ofelectrodes of the first capacitor is electrically connected to oneelectrode of a pair of electrodes of the second capacitor. The otherelectrode of the pair of electrodes of the first capacitor iselectrically connected to a gate of the transistor. The other electrodeof the pair of electrodes of the second capacitor is electricallyconnected to one of a source and a drain of the transistor. The secondswitch has a function of selecting conduction or non-conduction betweenthe second wiring and the gate of the transistor. The third switch has afunction of selecting conduction or non-conduction between the oneelectrode of the pair of electrodes of the first capacitor and the oneof the source and the drain of the transistor.

In the semiconductor device with the above structure, voltage appliedbetween the source and the gate of the transistor (hereinafter alsoreferred to as the driving transistor) can be corrected in anticipationof variations in threshold voltage. Thus, the drain current of thetransistor can be corrected. Further, the drain current can be suppliedto the load.

A semiconductor device according to one aspect of the present inventionincludes at least a transistor, a load, a first wiring, a second wiring,a first switch, a second switch, a third switch, a first capacitor, anda second capacitor. The first switch has a function of selectingconduction or non-conduction between the first wiring and one electrodeof a pair of electrodes of the first capacitor. The one electrode of thepair of electrodes of the first capacitor is electrically connected toone electrode of a pair of electrodes of the second capacitor. The otherelectrode of the pair of electrodes of the first capacitor iselectrically connected to a gate of the transistor. The other electrodeof the pair of electrodes of the second capacitor is electricallyconnected to the load and one of a source and a drain of the transistor.The second switch has a function of selecting conduction ornon-conduction between the second wiring and the gate of the transistor.The third switch has a function of selecting conduction ornon-conduction between the one electrode of the pair of electrodes ofthe first capacitor and the one of the source and the drain of thetransistor.

In the semiconductor device with the above structure, voltage appliedbetween the source and the gate of the transistor (hereinafter alsoreferred to as the driving transistor) can be corrected in anticipationof variations in threshold voltage. Thus, the drain current of thetransistor can be corrected. Further, the drain current can be suppliedto the load.

A given element or circuit can be used as the load. For example, alight-emitting element such as an EL element can be used as the load. Alight-emitting element such as an EL element emits light at luminancethat is proportional to the amount of current flowing between an anodeand a cathode of the light-emitting element.

In the case where a light-emitting element is used as the load, astructure of Type A or Type B, for example, can be employed.

(Type A)

In the semiconductor device according to one aspect of the presentinvention, one of the source and the drain of the transistor (thedriving transistor) can be electrically connected to the anode of thelight-emitting element. In that case, the transistor is an n-channeltransistor. In the semiconductor device according to one aspect of thepresent invention, the semiconductor device includes a unit (e.g., adriver circuit) having a function of controlling the potential of thefirst wiring. The unit (the driver circuit) controls the potential ofthe first wiring so that a period during which the potential of thefirst wiring is equal to or lower than the potential of the cathode ofthe light-emitting element is provided.

(Type B)

In the semiconductor device according to one aspect of the presentinvention, one of the source and the drain of the transistor (thedriving transistor) can be electrically connected to the cathode of thelight-emitting element. In that case, the transistor is a p-channeltransistor. In the semiconductor device according to one aspect of thepresent invention, the semiconductor device includes a unit (e.g., adriver circuit) having a function of controlling the potential of thefirst wiring. The unit (the driver circuit) controls the potential ofthe first wiring so that a period during which the potential of thefirst wiring is equal to or higher than the potential of the anode ofthe light-emitting element is provided.

Each of the first to third switches can be a transistor. The transistorcan have the same conductivity type as the driving transistor.

The semiconductor device according to one aspect of the presentinvention can be formed using a transistor whose channel formationregion includes an oxide semiconductor layer. Alternatively, thesemiconductor device can be formed using a transistor whose channelformation region includes single crystal silicon. Alternatively, thesemiconductor device can be formed using a transistor whose channelformation region includes polycrystalline silicon. Alternatively, thesemiconductor device can be formed using a transistor whose channelformation region includes amorphous silicon.

In other words, transistors with a variety of structures can be used asa transistor, without limitation to a certain type. For example, atransistor including single-crystal silicon or a thin film transistor(TFT) including a non-single-crystal semiconductor film typified byamorphous silicon, polycrystalline silicon, microcrystalline (alsoreferred to as microcrystal, nanocrystal, or semi-amorphous) silicon, orthe like can be used as a transistor.

Note that for example, a transistor including a compound semiconductor(e.g., SiGe, GaAs, and the like), an oxide semiconductor (e.g., ZnO,InGaZnO, indium zinc oxide, ITO (indium tin oxide), SnO, TiO, andAlZnSnO (AZTO), and InSnZnO), or the like; a thin film transistorobtained by thinning such a compound semiconductor or an oxidesemiconductor; or the like can be used as a transistor. Thus,manufacturing temperature can be lowered and for example, such atransistor can be formed at room temperature. Accordingly, thetransistor can be formed directly on a substrate having low heatresistance, such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or an oxide semiconductor can be used notonly for a channel portion of the transistor but also for otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used for a wiring, a resistor, a pixel electrode, alight-transmitting electrode, or the like. Since such an element can beformed at the same time as the transistor, cost can be reduced.

Note that for example, a transistor or the like including an organicsemiconductor or a carbon nanotube can be used as a transistor.

Note that a transistor with a multi-gate structure having two or moregate electrodes can be used. With the multi-gate structure, channelformation regions are connected in series; accordingly, a plurality oftransistors are connected in series. Thus, with the multi-gatestructure, the amount of off-state current can be reduced and thewithstand voltage (reliability) of the transistor can be increased.Alternatively, a transistor with the multi-gate structure can have aflat slope of voltage-current characteristics such that drain-sourcecurrent does not change much even if drain-source voltage changes whenthe transistor operates in a saturation region. By utilizing the flatslope of the voltage-current characteristics, an ideal current sourcecircuit or an active load with extremely high resistance can beachieved. Consequently, a differential circuit, a current mirrorcircuit, or the like having excellent properties can be fabricated.

For example, it is possible to use a transistor in which gate electrodesare provided above and below a channel. The structure where the gateelectrodes are provided above and below the channel is substantiallyequivalent to a circuit structure in which a plurality of transistorsare connected in parallel. Thus, the area of the channel region isincreased, so that the current value can be increased. Alternatively, byemploying the structure where gate electrodes are provided above andbelow the channel, a depletion layer is easily formed; thus, thesubthreshold swing (S value) can be reduced.

For example, it is possible to use a transistor with a structure where agate electrode is formed above or below a channel, a staggeredstructure, an inverted staggered structure, a structure where a channelis divided into a plurality of regions, a structure where channels areconnected in parallel or in series, or the like.

Note that for example, a transistor with a structure where an LDD regionis provided can be used as a transistor. By providing the LDD region,the amount of off-state current can be reduced or the withstand voltageof the transistor can be increased (reliability can be improved).Alternatively, by providing the LDD region, drain-source current doesnot fluctuate very much even when drain-source voltage fluctuates whenthe transistor operates in the saturation region, so that a flat slopeof voltage-current characteristics can be obtained.

The invention excluding content that is not specified in the drawingsand texts in this specification can be constructed. Alternatively, whenthe range of a value (e.g., the maximum and minimum values) isdescribed, the range may be freely narrowed or a value in the range maybe excluded, so that the invention can be specified by a range part ofwhich is excluded. In this manner, it is possible to specify the scopeof the present invention so that a conventional technology is excluded,for example

As a specific example, assuming that a circuit including first to fifthtransistors is illustrated in a circuit diagram, the invention can bedefined as the circuit that does not include a sixth transistor.Alternatively, the invention can be defined as the circuit that does notinclude a capacitor. Further, the invention can be constructed byspecifying that the circuit does not include a sixth transistor with aparticular connection. Alternatively, the invention can be constructedby specifying that the circuit does not include a capacitor with aparticular connection. For example, the invention can be defined byspecifying that the circuit does not include a sixth transistor whosegate is connected to a gate of the third transistor. Alternatively, forexample, the invention can be defined by specifying that the circuitdoes not include a capacitor whose first electrode is connected to thegate of the third transistor.

As another specific example, when the expression “a voltage preferablyranges from 3 V to 10 V” is used to describe a given value, theinvention can be defined, for example, by excluding the case where thevoltage is higher than or equal to −2 V and lower than or equal to 1 V.Alternatively, for example, the invention can be defined by excludingthe case where the voltage is higher than or equal to 13 V. Note thatfor example, it can be specified that the voltage is higher than orequal to 5 V and lower than or equal to 8 V in the invention. Moreover,it can be specified, in the invention, that the voltage is approximately9 V or that the voltage is higher than or equal to 3 V and lower than 9V and higher than 9 V and lower than or equal to 10 V

As another specific example, when the expression “a voltage ispreferably 10 V” is used to describe a given value, the invention can bedefined, for example, by excluding the case where the voltage is higherthan or equal to −2 V and lower than or equal to 1 V. Alternatively, forexample, the invention can be defined by excluding the case where thevoltage is higher than or equal to 13 V.

As another specific example, when the expression “a film is aninsulating film” is used to describe properties of a material, theinvention can be defined, for example, by excluding the case where theinsulating film is an organic insulating film. Alternatively, forexample, the invention can be defined by excluding the case where theinsulating film is an inorganic insulating film.

As another specific example, when the expression “a film is providedbetween A and B” is used to describe a layered structure, the inventioncan be defined, for example, by excluding the case where the film is astack of four or more layers. Alternatively, for example, the inventioncan be defined by excluding the case where a conductive film is providedbetween A and the film.

In one aspect of the present invention, it is possible to determinevoltage applied between a source and a gate of a driving transistordepending on the threshold voltage of the driving transistor. Thus, itis possible to provide a semiconductor device, a light-emitting device,or a display device that is less influenced by variations incharacteristics of transistors. It is possible to provide asemiconductor device, a light-emitting device, or a display device thatis less influenced by degradation of characteristics of a transistor. Itis possible to provide a semiconductor device, a light-emitting device,or a display device in which variations in luminance due to variationsin threshold voltage of driving transistors are reduced. It is possibleto provide a semiconductor device, a light-emitting device, or a displaydevice in which variations in luminance due to variations in mobility ofdriving transistors are reduced. It is possible to provide asemiconductor device, a light-emitting device, or a display device thatcorrectly operates even when using a normally-off transistor. It ispossible to provide a semiconductor device, a light-emitting device, ora display device that can obtain the threshold voltage of a normally-offtransistor. It is possible to provide a semiconductor device, alight-emitting device, or a display device that displays high-qualityimages. It is possible to provide a semiconductor device, alight-emitting device, or a display device that displays images withlittle unevenness. It is possible to provide a semiconductor device, alight-emitting device, or a display device in which a desired circuitcan be formed with a small number of transistors. It is possible toprovide a semiconductor device, a light-emitting device, or a displaydevice in which a desired circuit can be formed with a small number ofwirings. It is possible to provide a semiconductor device, alight-emitting device, or a display device that is less influenced bydegradation of a light-emitting element. It is possible to provide asemiconductor device, a light-emitting device, or a display device thatis manufactured in a small number of steps.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D each illustrate the structure of a semiconductor device;

FIGS. 2A to 2D each illustrate the structure of a semiconductor device;

FIGS. 3A to 3D each illustrate the structure of a semiconductor device;

FIGS. 4A to 4D each illustrate the structure of a semiconductor device;

FIG. 5A is a timing chart, and FIGS. 5B and 5C each illustrate theoperation of a semiconductor device;

FIGS. 6A to 6C each illustrate the operation of a semiconductor device;

FIGS. 7A to 7E each illustrate the operation of a semiconductor device;

FIGS. 8A to 8D each illustrate the structure of a semiconductor device;

FIGS. 9A to 9D each illustrate the structure of a semiconductor device;

FIGS. 10A and 10B each illustrate the structure of a semiconductordevice;

FIGS. 11A to 11D each illustrate the structure of a semiconductordevice;

FIGS. 12A to 12D each illustrate the structure of a semiconductordevice;

FIGS. 13A to 13D each illustrate the structure of a semiconductordevice;

FIGS. 14A to 14D each illustrate the structure of a semiconductordevice;

FIGS. 15A to 15D each illustrate the structure of a semiconductordevice;

FIGS. 16A to 16D each illustrate the structure of a semiconductordevice;

FIG. 17A is a timing chart, and FIGS. 17B and 17C each illustrate theoperation of a semiconductor device;

FIGS. 18A and 18B each illustrate the operation of a semiconductordevice;

FIGS. 19A to 19D each illustrate the operation of a semiconductordevice;

FIG. 20A is a timing chart, and FIG. 20B illustrates the operation of asemiconductor device;

FIGS. 21A to 21E each illustrate the structure of a semiconductordevice;

FIGS. 22A to 22E each illustrate the structure of a semiconductordevice;

FIGS. 23A and 23B each illustrate the structure of a semiconductordevice;

FIGS. 24A to 24D each illustrate the structure of a semiconductordevice;

FIGS. 25A to 25D each illustrate the structure of a semiconductordevice;

FIGS. 26A to 26D each illustrate the structure of a semiconductordevice;

FIGS. 27A to 27D each illustrate the structure of a semiconductordevice;

FIGS. 28A to 28D each illustrate the structure of a semiconductordevice;

FIGS. 29A to 29D each illustrate the structure of a semiconductordevice;

FIGS. 30A to 30D each illustrate the structure of a semiconductordevice;

FIGS. 31A to 31D each illustrate the structure of a semiconductordevice;

FIGS. 32A to 32D each illustrate the structure of a semiconductordevice;

FIGS. 33A to 33D each illustrate the structure of a semiconductordevice;

FIGS. 34A to 34D each illustrate the structure of a semiconductordevice;

FIGS. 35A to 35D each illustrate the operation of a semiconductordevice;

FIG. 36 illustrates the operation of a semiconductor device;

FIGS. 37A to 37D each illustrate the structure of a semiconductordevice;

FIGS. 38A to 38D each illustrate the structure of a semiconductordevice;

FIG. 39 illustrates the structure of a semiconductor device;

FIG. 40 illustrates the structure of a semiconductor device;

FIG. 41 illustrates the structure of a semiconductor device;

FIGS. 42A to 42D each illustrate the structure of a semiconductordevice;

FIGS. 43A to 43F each illustrate the structure of a semiconductordevice;

FIGS. 44A to 44F each illustrate the structure of a semiconductordevice;

FIGS. 45A to 45D each illustrate the structure of a semiconductordevice;

FIGS. 46A to 46D each illustrate the structure of a semiconductordevice;

FIGS. 47A to 47D each illustrate the structure of a semiconductordevice;

FIGS. 48A to 48D each illustrate the structure of a semiconductordevice;

FIGS. 49A to 49D each illustrate the structure of a semiconductordevice;

FIGS. 50A to 50D each illustrate the structure of a semiconductordevice;

FIGS. 51A to 51D each illustrate the structure of a semiconductordevice;

FIGS. 52A to 52D each illustrate the structure of a semiconductordevice;

FIGS. 53A to 53D each illustrate the structure of a semiconductordevice;

FIGS. 54A to 54D each illustrate the structure of a semiconductordevice;

FIGS. 55A to 55C each illustrate the structure of a semiconductordevice;

FIGS. 56A to 56C each illustrate the structure of a semiconductordevice;

FIGS. 57A to 57D each illustrate the structure of a semiconductordevice;

FIGS. 58A to 58D each illustrate the structure of a semiconductordevice;

FIGS. 59A to 59C each illustrate the structure of a semiconductordevice;

FIGS. 60A to 60C each illustrate the structure of a semiconductordevice;

FIGS. 61A to 61D each illustrate the structure of a semiconductordevice;

FIGS. 62A to 62D each illustrate the structure of a semiconductordevice;

FIGS. 63A to 63D each illustrate the structure of a semiconductordevice;

FIGS. 64A to 64D each illustrate the structure of a semiconductordevice;

FIGS. 65A to 65D each illustrate the structure of a semiconductordevice;

FIGS. 66A to 66D each illustrate the structure of a semiconductordevice;

FIGS. 67A to 67C each illustrate the structure of a semiconductordevice;

FIGS. 68A to 68C each illustrate the structure of a semiconductordevice;

FIGS. 69A to 69D each illustrate the structure of a semiconductordevice;

FIGS. 70A to 70D each illustrate the structure of a semiconductordevice;

FIGS. 71A to 71D each illustrate the structure of a semiconductordevice;

FIGS. 72A to 72D each illustrate the operation of a semiconductordevice;

FIGS. 73A to 73D each illustrate the structure of a semiconductordevice;

FIGS. 74A to 74F each illustrate the structure of a semiconductordevice;

FIGS. 75A to 75E each illustrate the structure of a semiconductordevice;

FIGS. 76A to 76G each illustrate the structure of a semiconductordevice;

FIG. 77 illustrates the structure of a semiconductor device;

FIG. 78 illustrates the structure of a semiconductor device;

FIG. 79 illustrates the structure of a semiconductor device;

FIGS. 80A and 80B illustrate the structure of a semiconductor device;

FIGS. 81A to 81C each illustrate the structure of a semiconductordevice;

FIGS. 82A to 82D each illustrate the structure of a semiconductordevice;

FIGS. 83A and 83B illustrate the structure of a semiconductor device;

FIGS. 84A and 84B each illustrate the structure of a semiconductordevice;

FIGS. 85A to 85F each illustrate an electronic device;

FIG. 86 illustrates the structure of a semiconductor device;

FIGS. 87A to 87D each illustrate the structure of a semiconductordevice;

FIG. 88A illustrates the structure of a semiconductor device, and FIG.88B is a timing chart;

FIG. 89 shows calculation results;

FIG. 90 shows calculation results; and

FIG. 91 illustrates an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be hereinafter described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that a variety of changesand modifications can be made without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be limited to the descriptions of the embodiments below. Instructures given below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and explanation thereof will not be repeated.

Note that what is described in one embodiment (or part of the content)can be applied to, combined with, or replaced with different content (orpart thereof) in the embodiment and/or content (or part thereof)described in another embodiment or other embodiments.

Note that the structure illustrated in a diagram (or part thereof) inone embodiment can be combined with the structure of another part of thediagram, the structure illustrated in a different diagram (or partthereof) in the embodiment, and/or the structure illustrated in adiagram (or part thereof)) in another embodiment or other embodiments.

Note that the size, thickness, and regions in the drawings areexaggerated for clarity in some cases. Thus, one aspect of theembodiment of the present invention is not limited to such scales.Alternatively, drawings schematically illustrate an ideal example Thus,one aspect of the embodiment of the present invention is not limited toshapes illustrated in the drawings and can include variations in shapedue to a fabrication technique or dimensional deviation, for example

Note that an explicit description “X and Y are connected” indicates thecase where X and Y are electrically connected, the case where X and Yare functionally connected, and the case where X and Y are directlyconnected. Here, each of X and Y denotes an object (e.g., a device, anelement, a circuit, a wiring, an electrode, a terminal, a conductivefilm, a layer, a display element, a light-emitting element, or a load).Accordingly, a connection relation other than those shown in drawingsand texts is also included without limitation to a predeterminedconnection relation, for example, the connection relation shown in thedrawings and the texts.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is conducting or not conducting (is turned on or off) todetermine whether current flows therethrough or not. Alternatively, theswitch has a function of selecting and changing a current path.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a DC-DCconverter, a step-up DC-DC converter, or a step-down DC-DC converter) ora level shifter circuit for changing the potential level of a signal; avoltage source; a current source; a switching circuit; an amplifiercircuit such as a circuit that can increase signal amplitude, the amountof current, or the like, an operational amplifier, a differentialamplifier circuit, a source follower circuit, or a buffer circuit; asignal generator circuit; a memory circuit; and/or a control circuit)can be connected between X and Y. When a signal output from X istransmitted to Y, it can be said that X and Y are functionally connectedeven if another circuit is provided between X and Y.

Note that an explicit description “X and Y are connected” means that Xand Y are electrically connected, X and Y are functionally connected,and X and Y are directly connected. That is, the explicit description “Xand Y are electrically connected” is the same as an explicit simpleexpression “X and Y are connected”.

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Note that it might be possible for those skilled in the art toconstitute one aspect of the invention even when portions to which allterminals of an active element (e.g., a transistor or a diode), apassive element (e.g., a capacitor or a resistor), or the like areconnected are not specified. In other words, even when such portions arenot specified, one aspect of the present invention can be clear and itcan be determined that one aspect of the present invention is disclosedin this specification and the like in some cases. In particular, in thecase where the number of portions to which the terminal is connected maybe plural, it is not necessary to specify the portions to which theterminal is connected. Therefore, it might be possible to constitute oneaspect of the invention by specifying portions to which only some ofterminals of an active element (e.g., a transistor or a diode), apassive element (e.g., a capacitor or a resistor), or the like areconnected.

Note that it might be possible for those skilled in the art to specifythe invention when at least where a circuit is to be connected(“connection point”) is specified. Alternatively, it might be possiblefor those skilled in the art to specify the invention when at least afunction of a circuit is specified. In other words, when a function of acircuit is specified, one aspect of the present invention can be clearand it can be determined that one aspect of the present invention isdisclosed in this specification and the like in some cases. Therefore,when a connection point of a circuit is specified, the circuit isdisclosed as one aspect of the invention even if a function is notspecified, and one aspect of the invention can be constructed.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one aspect of the invention even if a connection point isnot specified, and one aspect of the invention can be constructed.

Note that various people can implement one aspect of the embodiment ofthe present invention. However, different people may be involved in theimplementation of the invention. For example, in the case of atransmission/reception system, the following case is possible: Company Amanufactures and sells transmitting devices, and Company B manufacturesand sells receiving devices. As another example, in the case of alight-emitting device including a TFT and a light-emitting element, thefollowing case is possible: Company A manufactures and sellssemiconductor devices including TFTs, and Company B purchases thesemiconductor devices, provides light-emitting elements for thesemiconductor devices, and completes light-emitting devices.

In such a case, one aspect of the invention can be constituted so that apatent infringement can be claimed against each of Company A and CompanyB. That is, one aspect of the invention with which a patent infringementsuit can be filed against Company A or Company B is clear and can beregarded as being disclosed in this specification or the like. Forexample, in the case of a transmission/reception system, one aspect ofthe invention can be constituted by only a transmitting device and oneaspect of the invention can be constituted by only a receiving device.Those embodiments of the invention are clear and can be regarded asbeing disclosed in this specification or the like. As another example,in the case of a light-emitting device including a TFT and alight-emitting element, one aspect of the invention can be constitutedby only a semiconductor device including a TFT, and one aspect of theinvention can be constituted by only a light-emitting device including aTFT and a light-emitting element. Those embodiments of the invention areclear and can be regarded as being disclosed in this specification orthe like.

Embodiment 1

One aspect of the present invention can be used not only as a pixelincluding a light-emitting element but also as a variety of circuits.For example, it can be used as either an analog circuit or a circuitfunctioning as a current source. First, in this embodiment, examples ofa basic principle of a circuit disclosed in the present invention aredescribed.

A semiconductor device according to one aspect of the present inventionincludes at least, for example, a transistor and an element having afunction of releasing an electric charge stored between a gate and asource of the transistor while the potential of the gate is fixed. Withthis element, the semiconductor device according to one aspect of thepresent invention can correct variations in drain current due to thethreshold voltage, mobility, or the like of the transistor.

A circuit 100 in FIG. 1A is a semiconductor device according to oneaspect of the present invention. The circuit 100 includes a switch 11, aswitch 12, a switch 13, a transistor 101, a capacitor 102, and acapacitor 103. Note that FIG. 1A shows the case where the transistor 101is an n-channel transistor.

Specifically, in FIG. 1A, the switch 11 has a function of controllingthe conduction between a wiring 21 and one electrode (terminal) of thecapacitor 102 or 103. The switch 12 has a function of controlling theconduction between a wiring 22 and the other electrode (terminal) of thecapacitor 102 and between the wiring 22 and a gate of the transistor101. The switch 13 has a function of controlling the conduction betweenone of a source and a drain of the transistor 101 or the other electrode(terminal) of the capacitor 103 and the one electrode of the capacitor102 or 103. The other of the source and the drain of the transistor 101is connected to a wiring 23. The one of the source and the drain of thetransistor 101 or the other electrode (terminal) of the capacitor 103 isconnected to a wiring 24.

Note that the terms “source” (source terminal, source region, or sourceelectrode) and “drain” (drain terminal, drain region, or drainelectrode) of a transistor interchange with each other depending on thepolarity of the transistor or the levels of potentials applied to thesource and the drain. In general, as for a source and a drain in ann-channel transistor, one to which a lower potential is applied iscalled a source, and one to which a higher potential is applied iscalled a drain. Further, as for a source and a drain in a p-channeltransistor, one to which a lower potential is supplied is called adrain, and one to which a higher potential is supplied is called asource. In this specification, although connection relation of thetransistor is described assuming that the source and the drain are fixedin some cases for convenience, actually, the names of the source and thedrain interchange with each other depending on the relation of thepotentials. Thus, a portion which serves as a source or a portion whichserves as a drain is not referred to as a source or a drain in somecases. In that case, one of the source and the drain might be referredto as a first terminal, a first electrode, or a first region, and theother of the source and the drain might be referred to as a secondterminal, a second electrode, or a second region, for example

Note that a switch is an element having a function of operating byswitching conduction and non-conduction between its terminals and afunction of determining whether or not current flows between them.Alternatively, the switch has a function of selecting and changing acurrent path. For example, the switch has a function of determiningwhether current can flow through a path 1 or a path 2 and switching thepaths. For example, an electrical switch or a mechanical switch can beused as the switch. Specifically, the switch may be formed using atransistor, a diode, or a switch formed by a micro electro mechanicalsystem (MEMS) technology, such as a digital micromirror device (DMD).Alternatively, the switch may be a logic circuit in which transistorsare combined. In the case of employing a transistor as the switch, thereis no particular limitation on the polarity (conductivity type) of thetransistor. Note that a transistor with small off-state current ispreferably used and the polarity of the transistor is preferablyselected in accordance with an input potential.

Examples of the transistor with small off-state current are a transistorprovided with an LDD region, a transistor with a multi-gate structure,and a transistor whose channel formation region contains an oxidesemiconductor. In the case where a combination of transistors operatesas a switch, a complementary switch may be employed by using both ann-channel transistor and a p-channel transistor. A complementary switchachieves appropriate operation even when a potential input to the switchis changed relative to an output potential.

Note that, when a transistor is used as a switch, the switch includes aninput terminal (one of a source and a drain), an output terminal (theother of the source and the drain), and a terminal for controllingconduction (gate) in some cases. On the other hand, when a diode is usedas a switch, the switch does not have a terminal for controllingconduction in some cases. Therefore, when a diode is used as a switch,the number of wirings for controlling terminals can be reduced ascompared to the case of using a transistor.

Note that, for example, a transistor with a structure where gates areprovided above and below a channel formation region can be used as atransistor. By providing the gates above and below a semiconductor film,a circuit structure where a plurality of transistors is connected inparallel is provided. Thus, a channel formation region is increased, sothat the amount of current can be increased. By employing the structurewhere the gates are provided above and below the channel formationregion, a depletion layer is easily formed; thus, subthreshold swing (Svalue) can be improved.

Note that, for example, a transistor with a structure where a sourceelectrode or a drain electrode overlaps with a channel formation region(or part thereof) can be used as a transistor. By employing thestructure where the source electrode or the drain electrode overlapswith the channel formation region (or part thereof), unstable operationdue to electric charge accumulated in part of the channel formationregion can be prevented.

Note that the capacitor 102 or 103 may have a structure where aninsulating film or an organic film is sandwiched between wirings,semiconductor layers, electrodes, or the like, for example.

Note that the circuit 100 in FIG. 1A may include a load 104 asillustrated in FIG. 1B. In the circuit 100 in FIG. 1B, the load 104 isconnected between one of the source and the drain of the transistor 101or the other electrode of the capacitor 103 and the wiring 24.

Note that, in this specification, a load means an object having arectifying property, an object having capacitance, an object havingresistance, a circuit including a switch, a current source circuit, orthe like. For example, a load having a rectifying property hascurrent-voltage characteristics showing different resistance valuesdepending on the direction of an applied bias, and has an electricproperty which allows most current to flow only in one direction.Specifically, the load 104 may be a display element (e.g., a liquidcrystal element and an EL element), a light-emitting element (an EL(electroluminescence) element, e.g., an EL element containing organicand inorganic materials, an organic EL element, or an inorganic ELelement), and an LED (e.g., a white LED, a red LED, a green LED, or ablue LED), a transistor (a transistor which emits light in accordancewith the amount of current), an electron emitter, a part of a displayelement or a light-emitting element (e.g., a pixel electrode, an anode,and a cathode), or the like.

FIG. 1C illustrates the structure of a circuit 100 which uses alight-emitting element 104 a as the load 104. FIG. 1C illustrates thecase where an anode of the light-emitting element 104 a is connected toone of the source and the drain of the transistor 101 or the otherelectrode of the capacitor 103, and a cathode of the light-emittingelement 104 a is connected to the wiring 24.

FIG. 1D illustrates the structure of a circuit 100 which uses alight-emitting element 104 b as the load 104. FIG. 1D illustrates thecase where a cathode of the light-emitting element 104 b is connected toone of the source and the drain of the transistor 101 or the otherelectrode of the capacitor 103, and an anode of the light-emittingelement 104 b is connected to the wiring 24. Note that FIG. 1D shows thecase where the transistor 101 is a p-channel transistor.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 1A to 1D.

Semiconductor devices illustrated in FIGS. 2A to 2D include, in additionto the circuits 100 in FIGS. 1A to 1D, a circuit 201 having a functionof supplying a constant voltage or a signal to the wiring 21, a circuit202 having a function of supplying a constant voltage or a signal to thewiring 22, a circuit 203 having a function of supplying a constantvoltage or a signal to the wiring 23, and a circuit 204 having afunction of supplying a constant voltage or a signal to the wiring 24.

Specifically, the circuit 201 has a function of supplying a potentialVi1 or a potential Vsig to the wiring 21. An example of the circuit 201is a source driver (a signal line driver circuit). Accordingly, thewiring 21 has a function of transmitting or supplying the potential Vi1and/or the potential Vsig. The wiring 21 functions as a video signalline. Alternatively, the wiring 21 functions as an initialization line.

The potential Vi1 is a potential for initializing the potential of eachnode in the circuit 100, for example. Alternatively, the potential Vi1is a potential for supplying electric charge to the capacitor 102, forexample. Alternatively, the potential Vi1 is a potential for turning onthe transistor 101, for example Note that the potential Vi1 ispreferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential Vi1 may varylike a pulse signal.

Note that, as an example, the potential Vi1 is supplied to the circuit100 before the potential Vsig is supplied to the circuit 100.

The potential Vsig is a potential for controlling the amount of thedrain current of the transistor 101. In the case of the semiconductordevice illustrated in FIG. 2B, the drain current is supplied to the load104. In the case of the semiconductor device illustrated in FIG. 2C, thedrain current is supplied to the light-emitting element 104 a. In thecase of the semiconductor device illustrated in FIG. 2D, the draincurrent is supplied to the light-emitting element 104 b. For example,when the drain current of the transistor 101 is kept at a constantvalue, the level of the potential Vsig is set constant. In contrast, forexample, when the drain current of the transistor 101 is not set at aconstant value, the level of the potential Vsig is changed with time. Asan example, the potential Vsig is a video signal and/or an analogsignal. However, one aspect of the embodiment of the present inventionis not limited to this; the potential Vsig may be a constant potential.

The circuit 202 has a function of supplying a potential Vi2 to thewiring 22. An example of the circuit 202 is a power supply circuit.Accordingly, the wiring 22 has a function of transmitting or supplyingthe potential Vi2. Alternatively, the wiring 22 functions as aninitialization line. Note that the potential of the wiring 22 ispreferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 22may vary like a pulse signal.

The potential Vi2 is a potential for initializing the potential of eachnode (particularly the gate of the transistor 101) in the circuit 100.In the case in FIG. 2C, the potential Vi2 is preferably lower than orequal to the potential of the wiring 24. Thus, current flowing to thelight-emitting element 104 a can be reduced. In the case in FIG. 2D, thepotential Vi2 is preferably higher than or equal to the potential of thewiring 24. Thus, current flowing to the light-emitting element 104 b canbe reduced. However, the potential of the potential Vi2 is not limitedto this. Note that the potential Vi2 is preferably constant; however,one aspect of the embodiment of the present invention is not limitedthereto. The potential Vi2 may vary like a pulse signal.

Note that the wiring 22 may be connected to another wiring or a wiringincluded in another circuit 100. Thus, the number of wirings can bereduced.

The circuit 203 has a function of supplying a supply voltage (a highsupply potential or a low supply potential), e.g., a potential VDD or apotential VSS to the wiring 23. Alternatively, the circuit 203 has afunction of supplying a signal to the wiring 23. Examples of the circuit203 are a power supply circuit, a pulse output circuit, and a gatedriver circuit. Accordingly, the wiring 23 has a function oftransmitting or supplying a power supply potential or a signal.Alternatively, the wiring 23 has a function of supplying current to thetransistor 101. Alternatively, the wiring 23 has a function of supplyingcurrent to the load 104. The wiring 23 functions as a power supply line.Alternatively, the wiring 23 functions as a current supply line. Notethat the potential of the wiring 23 is preferably constant; however, oneaspect of the embodiment of the present invention is not limitedthereto. The potential of the wiring 23 may vary like a pulse signal.For example, the potential of the wiring 23 may be a potential at whichnot only forward bias voltage but also reverse bias voltage is appliedto the load 104.

The circuit 204 has a function of supplying, for example, a supplyvoltage (a low supply potential or a high supply potential), e.g., apotential Vcat to the wiring 24. An example of the circuit 204 is apower supply circuit. Accordingly, the wiring 24 has a function oftransmitting or supplying a power supply potential. Alternatively, thewiring 24 has a function of supplying current to the load 104.Alternatively, the wiring 24 has a function of supplying current to thetransistor 101. The wiring 24 functions as a common line. Alternatively,the wiring 24 functions as a negative line. Alternatively, the wiring 24functions as a positive line. Note that the potential of the wiring 24is preferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 24may vary like a pulse signal. For example, the potential of the wiring24 may be a potential at which not only forward bias voltage but alsoreverse bias voltage is applied to the load 104.

A difference between the potential VDD and the potential Vcat determinesthe direction of the drain current of the transistor 101. For example,when the potential VDD is higher than the potential Vcat, current flowsfrom the wiring 23 to the wiring 24. When the potential of the wiring 23is equal to the potential VSS and lower than the potential Vcat, currentflows from the wiring 24 to the wiring 23.

Note that in FIGS. 2A to 2D, the semiconductor devices each include, inaddition to the circuit 100, the circuits 201, 202, 203, and 204, as anexample. However, a semiconductor device according to one aspect of thepresent invention does not necessarily include all of the circuits 201,202, 203, and 204, and may include only one or some of these circuits.

The transistor 101 functions as at least a current source, for example.Accordingly, for example, the transistor 101 has a function of supplyingsubstantially constant current even when the level of voltage appliedacross both ends (between the source and the drain) of the transistor101 is changed. Alternatively, for example, the transistor 101 has afunction of supplying substantially constant current to the load 104even when the potential of the load 104 is changed. Alternatively, forexample, the transistor 101 has a function of supplying substantiallyconstant current even when the potential of the wiring 23 is changed.

Note that one aspect of the embodiment of the present invention is notlimited to this; the transistor 101 does not necessarily function as acurrent source. For example, the transistor 101 may function as aswitch.

Note that there is a voltage source as a power source different from acurrent source. The voltage source has a function of supplying constantvoltage even when current flowing through a circuit connected to thevoltage source is changed. Accordingly, the voltage source and thecurrent source each have a function of supplying voltage and current.However, the function of the voltage source and the function of thecurrent source are different in what is supplied at a constant leveleven when one factor is changed. The current source has a function ofsupplying constant current event when voltage across both ends ischanged. The voltage source has a function of supplying constant voltageeven when current is changed.

Note that FIGS. 1A to 1D and the like each illustrate a circuitstructure example; thus, a transistor can be additionally provided. Incontrast, for each node in FIGS. 1A to 1D and the like, it is possiblenot to provide an additional transistor, switch, passive element, or thelike. For example, it is possible not to provide an additionaltransistor that is directly connected to a node where terminals ofswitches are connected to each other, a node where terminals of atransistor are connected to each other, and/or a node where terminals ofa load are connected to each other. Accordingly, for example, it ispossible to directly connect only the transistor 101 to a node where theload 104, the transistor 101, the capacitor 103, and the switch 13 areconnected to each other, and it is possible not to directly connectanother transistor to the node.

Thus, a circuit can be formed with a small number of transistors in thecase where an additional transistor is not provided.

Note that in the circuits 100 in FIGS. 1A to 1D and FIGS. 2A to 2D, theswitch 11, the switch 12, and the switch 13 may be transistors.

FIGS. 3A to 3D illustrate the structures of circuits 100 whichcorrespond to the circuits 100 in FIGS. 1A to 1D each using a transistor11 t as the switch 11, a transistor 12 t as the switch 12, and atransistor 13 t as the switch 13. FIGS. 3A to 3C illustrate the casewhere the transistor 11 t, the transistor 12 t, and the transistor 13 tare all n-channel transistors. FIG. 3D illustrates the case where thetransistor 11 t, the transistor 12 t, and the transistor 13 t are allp-channel transistors. When the transistor 11 t, the transistor 12 t,and the transistor 13 t have the same polarity, these transistors can bemanufactured in fewer steps. However, one aspect of the embodiment ofthe present invention is not limited to this; these transistors may havedifferent polarities.

Note that in FIGS. 3A to 3D, a gate of the transistor 11 t is connectedto a wiring 31. The transistor 11 t is turned on or off in response tothe potential supplied to the wiring 31. A gate of the transistor 12 tis connected to a wiring 32. The transistor 12 t is turned on or off inresponse to the potential supplied to the wiring 32. A gate of thetransistor 13 t is connected to a wiring 33. The transistor 13 t isturned on or off in response to the potential supplied to the wiring 33.Therefore, it is preferable that the potentials of the wirings 31 to 33be pulsed potentials and not constant; however, one aspect of theembodiment of the present invention is not limited to this.Alternatively, the wirings 31 to 33 each function as a gate signal line(a gate line), a selection signal line, or a scan line.

Note that at least two of these wirings 31 to 33 can be connected toeach other. Alternatively, at least one of these wirings 31 to 33 can beconnected to at least one of the wirings 31 to 33 in another circuit100.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 3A to 3D.

Semiconductor devices illustrated in FIGS. 4A to 4D include, in additionto the circuits 100 in FIGS. 3A to 3D, a circuit 205 having a functionof supplying a constant voltage or a signal to the wiring 31, a circuit206 having a function of supplying a constant voltage or a signal to thewiring 32, and a circuit 207 having a function of supplying a constantvoltage or a signal to the wiring 33. Examples of the circuits 205, 206,and 207 include gate drivers (scan line driver circuits).

Note that the circuits 201, 202, 203, 204, 205, 206, and 207 may be thesame circuit or different circuits.

Note that in FIGS. 4A to 4D, the semiconductor devices each include, inaddition to the circuit 100, the circuits 205, 206, and 207, as anexample. However, a semiconductor device according to one aspect of thepresent invention does not necessarily include all of the circuits 205,206, and 207, and may include only one or some of these circuits.

FIG. 37A illustrates the structure of a circuit 100 corresponding to thecircuit 100 in FIG. 3C, in which the transistors 101, 11 t, and 13 t aren-channel transistors and the transistor 12 t is a p-channel transistor.FIG. 37B illustrates the structure of a circuit 100 corresponding to thecircuit 100 in FIG. 3D, in which the transistors 101, 11 t, and 13 t arep-channel transistors and the transistor 12 t is an n-channeltransistor. As described above, transistors of a variety of polaritiescan be used.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 37A and 37B.

Semiconductor devices illustrated in FIGS. 37C and 37D include, inaddition to the circuits 100 in FIGS. 37A and 37B, a circuit 205 havinga function of supplying a constant voltage or a signal to the wiring 31,a circuit 206 having a function of supplying a constant voltage or asignal to the wiring 32, and a circuit 207 having a function ofsupplying a constant voltage or a signal to the wiring 33.

Note that in FIGS. 37C and 37D, the semiconductor devices each include,in addition to the circuit 100, the circuits 205, 206, and 207, as anexample. However, a semiconductor device according to one aspect of thepresent invention does not necessarily include all of the circuits 205,206, and 207, and may include only one or some of these circuits.

In many cases, the transistor 101 operates in a saturation region whencurrent flows therethrough. Therefore, in FIGS. 3A to 3D, FIGS. 4A to4D, and FIGS. 37A to 37D, the transistor 101 preferably has a longerchannel length or gate length than the transistor 11 t, the transistor12 t, and/or the transistor 13 t. When the channel length or the gatelength is increased, characteristics in a saturation region have a flatslope; accordingly, a kink effect can be reduced. The channel length orgate length of the transistor 101 is preferably 5 or more times, furtherpreferably 10 or more times that of the transistor 11 t, the transistor12 t, and/or the transistor 13 t. As an example, the channel length orgate length of the transistor 101 is preferably 10 μm or more, morepreferably 20 μm or more. Alternatively, the channel width or gate widthof the transistor 101 is larger than that of the transistor 11 t, thetransistor 12 t, and/or the transistor 13 t, so that much current flowsthrough the transistor 101 even in a saturation region. The channelwidth or the gate width of the transistor 101 is preferably 5 or moretimes, further preferably 10 or more times that of the transistor 11 t,the transistor 12 t, and/or the transistor 13 t. The channel width orthe gate width of the transistor 101 is preferably 20 μm or more, morepreferably 30 μm or more. Note that one aspect of an embodiment of thepresent invention is not limited thereto.

The following describes the operation of a semiconductor deviceaccording to one aspect of the present invention, taking the circuit 100illustrated in FIG. 1C as an example.

The operation of the circuit 100 illustrated in FIG. 1C can be mainlydivided into a first operation, a second operation, a third operation, afourth operation, and a fifth operation. Note that one aspect of anembodiment of the present invention is not limited thereto, and anotheroperation can be added or part of the operation can be omitted.

FIG. 5A is an example of a timing chart showing the operations of theswitch 11, the switch 12, and the switch 13, the potential of the wiring21, and the gate-source voltage of the transistor 101 (Vgs101) in thecircuit 100 illustrated in FIG. 1C.

First, the first operation in the period T11 is described. In the periodT11, as illustrated in FIG. 5A, the switch 11, the switch 12, and theswitch 13 are on. Further, the potential Vi1 is supplied to the wiring21. Thus, in the period T11, as illustrated in FIG. 5B, the voltageVi2−Vi1 is applied to the capacitor 102, the potential of the anode ofthe light-emitting element 104 a becomes the potential Vi1, and thegate-source voltage Vgs101 of the transistor 101 becomes the voltageVi2−Vi1. In other words, the transistor 101 and the capacitor 102 areinitialized.

Note that in the circuit 100 in FIG. 1C, the potential Vi2 is preferablyhigher than the sum of the potential Vi1 and the threshold voltage Vthof the transistor 101. In other words, the potential Vi2 and thepotential Vi1 are preferably potentials with which the transistor 101 isturned on. Moreover, the sum of the potential Vi1 and a thresholdvoltage Vthe of the light-emitting element 104 a (a voltage at which thelight-emitting element 104 a starts to emit light) is preferably lowerthan the potential Vcat. For example, the potential Vi1 is preferablylower than or equal to the potential Vcat. When the potential Vi1 islower than the potential Vcat, the light-emitting element 104 a isreverse-biased, so that degradation of the light-emitting element 104 acan be reduced or a short-circuited portion can be repaired. Further, avalue obtained by subtracting the threshold voltage Vthe of thelight-emitting element 104 a from the potential Vi2 is preferably lowerthan the potential Vcat. Note that, as an example, the threshold voltageVthe is hereinafter assumed to be 0.

Next, the second operation in the period T12 is described. In the periodT12, as illustrated in FIG. 5A, the switch 11 is off, and the switch 12and the switch 13 are on. When the switch 11 is turned off, electriccharge accumulated in the capacitor 102 is released through thetransistor 101, and the potential of the source of the transistor 101 israised. Then, when the transistor 101 is turned off, the release of theelectric charge from the capacitor 102 is stopped. The threshold voltageVth of the transistor 101 is eventually held in the capacitor 102. Thus,in the period T12, as illustrated in FIG. 5C, the threshold voltage Vthis held in the capacitor 102, the potential of the anode of thelight-emitting element 104 a becomes the potential Vi2−Vth, and thegate-source voltage Vgs101 of the transistor 101 becomes the thresholdvoltage Vth. That is, the threshold voltage Vth of the transistor 101can be acquired.

Note that in some cases, it takes a very long time for Vgs101 to beequal to the threshold voltage Vth of the transistor 101. Accordingly,in many cases, an operation is performed while Vgs101 is not completelylowered to the threshold voltage Vth. That is, in many cases, the periodT12 is terminated while Vgs101 is slightly higher than the thresholdvoltage Vth. In other words, at the termination of the period T12,Vgs101 becomes voltage based on the threshold voltage.

Note that the second operation can be performed regardless of whetherthe threshold voltage Vth of the transistor 101 is positive voltage ornegative voltage. This is because the potential of the source of thetransistor 101 can be raised until the transistor 101 is turned off. Inother words, when the potential of the source of the transistor 101becomes higher than potential of the gate of the transistor 101, thetransistor 101 can be eventually turned off and Vgs101 can become Vth.Thus, the second operation can be performed without problems regardlessof whether the transistor 101 is an enhancement (normally-off)transistor or a depletion (normally-on) transistor.

Note that when the potential of the anode of the light-emitting element104 a becomes high, it is preferable that current does not flow to thelight-emitting element 104 a. For this purpose, the potential Vi2 ispreferably a low potential so that current does not flow to thelight-emitting element 104 a. Note that one aspect of an embodiment ofthe present invention is not limited thereto. If it is possible not tosupply current to the light-emitting element 104 a when a switchprovided in series with the light-emitting element 104 a is turned off,the potential Vi2 may be at a high value.

Next, the third operation in the period T13 is described. In the periodT13, as illustrated in FIG. 5A, the switch 11 and the switch 13 are on,and the switch 12 is off. In addition, as an example, the potential Vi1is supplied to the wiring 21. Thus, in the period T13, as illustrated inFIG. 6A, the threshold voltage Vth (the voltage based on Vth) is held inthe capacitor 102, the potential of the anode of the light-emittingelement 104 a becomes the potential Vi1, the potential of the gate ofthe transistor 101 becomes the potential Vi1+Vth (or the voltage basedon Vth), and the gate-source voltage Vgs101 of the transistor 101becomes the voltage Vth (or the voltage based on Vth). Thus, thepotential of the anode of the light-emitting element 104 a or thepotential of the source of the transistor 101 can be initialized.

Note that the third operation is not necessarily performed; the fourthoperation described below may be performed after the second operation.

Note that the potential of the wiring 21 in the period T13 is notlimited to the potential Vi1, and may be another potential (e.g., apotential Vi3). However, when the potential of the wiring 21 in theperiod T13 is the potential Vi1, the structure of the circuit 201 can besimplified. In the case where a plurality of circuits 100 is connectedto the wiring 21, when the potential of the wiring 21 is the potentialVi1, one circuit 100 performs the operation of the period T11, andanother circuit 100 performs the operation of the period T13, resultingin efficient use of operation period.

Next, the fourth operation in the period T14 is described. In the periodT14, as illustrated in FIG. 5A, the switch 11 is on, and the switch 12and the switch 13 are off. In addition, the potential Vsig is suppliedto the wiring 21. Thus, in the period T14, as illustrated in FIG. 6B,the threshold voltage Vth (or the voltage based on Vth) is held in thecapacitor 102, a voltage Vsig−Vi1−Vα is held in the capacitor 103, thepotential of the anode of the light-emitting element 104 a becomes apotential Vi1+Vα, the potential of the gate of the transistor 101becomes the potential Vsig+Vth, and the gate-source voltage Vgs101 ofthe transistor 101 becomes a voltage Vsig+Vth−Vi1−Vα. Thus, thepotential Vsig can be supplied to the capacitor 103. Alternatively, thesum of the voltage across the capacitor 102 and the voltage across thecapacitor 103 can be equal to the gate-source voltage of the transistor101.

Note that the potential Vα in the fourth operation varies when the anodeof the light-emitting element 104 a enters into an electrically floatingstate. The value of the potential Vα depends on the ratio between thecapacitance of the light-emitting element 104 a and the capacitance ofthe capacitor 102 and capacitor 103 if the transistor 101 is off.However, depending on the level of the potential Vsig, the transistor101 might be turned on and electric charge flows into the anode of thelight-emitting element 104 a through the transistor 101. Thus, the valueof the potential Vα depends not only on the capacitance ratio but alsoon the electric charge flowing into the anode of the light-emittingelement 104 a.

Here, in order that the gate-source voltage Vgs may be close to an idealvalue, i.e., the voltage Vsig+Vth−Vi1, the circuit is preferablydesigned to reduce the potential Vα. Specifically, if the capacitance ofthe light-emitting element 104 a is sufficiently greater than those ofthe capacitor 102 and the capacitor 103, the gate-source voltage Vgs canbe close to the ideal value.

Therefore, the capacitance of the capacitor 103 is preferably lower thanthe parasitic capacitance of the load 104 (the light-emitting element104 a), more preferably ½ or less, still more preferably ⅕ or less ofthe capacitance of the load 104. Alternatively, the area of theelectrodes of the capacitor 103 is preferably smaller than the area ofthe electrodes of the load 104 (the light-emitting element 104 a), morepreferably ½ or less of the area of the electrodes of the load 104,still more preferably ⅕ or less of the area of the electrodes of theload 104. Note that one aspect of an embodiment of the present inventionis not limited thereto.

In order that the gate-source voltage Vgs101 may be close to an idealvalue, it is preferable to reduce electric charge Q flowing into theanode of the light-emitting element 104 a. To reduce the electric chargeQ, the period T14 is preferably as short as possible. Note that asdescribed above, if the potential Vsig is supplied to the wiring 21 inadvance in the period T13, the potential of the gate of the transistor101 can be set close to the potential Vsig+Vth in a short time after theswitch 11 is turned on in the period T14. This is preferable becausethis shortens the period T14, and thus reduces the electric charge Q.

Therefore, the length of the period T14 is preferably smaller than thatof the period T11, the period T12, and/or the period T13. The length ofthe period T13 is preferably ⅔ or less, more preferably ½ or less of theperiod T11, the period T12, and/or the period T13. Note that one aspectof an embodiment of the present invention is not limited thereto.

Note that the electric charge Q is preferably low as described above;however, in the case where variations in mobility between thetransistors 101 are significant, the electric charge Q may produce theeffect of suppressing such variations in mobility. The reason will bedescribed below.

The electric charge Q is the amount of electric charge which flows fromthe drain to the source of the transistor 101 in the period T14.Accordingly, the electric charge Q is increased as the mobility of thetransistor 101 increases. As the electric charge Q is increased, thegate-source voltage Vgs 101 of the transistor 101 at the time when thelight-emitting element 104 a emits light is reduced. In other words, asthe mobility of the transistor 101 increases, correction is made by theelectric charge Q so that the value of current supplied to thelight-emitting element 104 a may be small, whereas, as the mobility ofthe transistor 101 decreases, correction is made by the electric chargeQ so that the value of current supplied to the light-emitting element104 a may not be so small. Thus, variations in mobility can besuppressed by the electric charge Q.

Note that the capacitance of the capacitor 102 is preferably higher thanthe parasitic capacitance of the gate of the transistor 101, morepreferably 2 or more times the parasitic capacitance of the gate of thetransistor 101, still more preferably 5 or more times the parasiticcapacitance of the gate of the transistor 101. Alternatively, the areaof the electrodes of the capacitor 102 is preferably larger than thearea of a channel region of the transistor 101, more preferably 2 ormore times the area of the channel region of the transistor 101, stillmore preferably 5 or more times the area of the channel region of thetransistor 101. Alternatively, the area of the electrodes of thecapacitor 102 is preferably larger than the area of the gate electrodeof the transistor 101, more preferably 2 or more times the area of thegate electrode of the transistor 101, still more preferably 5 or moretimes the area of the gate electrode of the transistor 101. Accordingly,when the potential Vsig is input and voltage is divided by the capacitor102 and the gate capacitance of the transistor, a decrease in voltage ofthe capacitor 102 can be reduced. Note that one aspect of the embodimentof the present invention is not limited thereto.

Note that the capacitance of the capacitor 102 is preferably equal to orhigher than the capacitance of the capacitor 103. The difference betweenthe capacitance of the capacitor 102 and the capacitance of thecapacitor 103 is preferably ±20% or lower, more preferably ±10% orlower. Alternatively, the area of the electrodes of the capacitor 102 ispreferably equal to or larger than the area of the electrodes of thecapacitor 103. Accordingly, the semiconductor device can perform optimumoperation without changing the layout area. Note that one aspect of theembodiment of the present invention is not limited thereto.

The fifth operation in the period T15 is described. In the period T15,as illustrated in FIG. 5A, the switch 11, the switch 12, and the switch13 are off. Thus, in the period T15, as illustrated in FIG. 6C, thethreshold voltage Vth is held in the capacitor 102, the voltageVsig−Vi1−Vα is held in the capacitor 103, the potential of the anode ofthe light-emitting element 104 a becomes the potential Vel, thepotential of the gate of the transistor 101 becomes the potentialVsig+Vth−Vi1−Vα+Vel and the gate-source voltage Vgs101 of the transistor101 becomes the voltage Vsig+Vth−Vi1−Vα. Thus, current based on thepotential Vsig can flow to the light-emitting element 104 a, so that thelight-emitting element 104 a can emit light at luminance based on thepotential Vsig.

Note that the potential Vel occurs when current is fed to thelight-emitting element 104 a through the transistor 101. Specifically,the potential Vel is between the potential VDD and the potential Vcat.

In the fifth operation, the gate-source voltage Vgs101 of the transistor101 can be set to the voltage Vsig+Vth−Vi1−Vα by taking the thresholdvoltage Vth of the transistor 101 into consideration. Consequently,variations in threshold voltage Vth between the transistors 101 can beprevented from adversely affecting the value of a current supplied tothe light-emitting elements 104 a. Alternatively, even if the transistor101 deteriorates and the threshold voltage Vth is changed, the change inthe threshold voltage Vth can be prevented from adversely affecting thevalue of a current supplied to the light-emitting element 104 a.Therefore, high-quality images with less unevenness can be displayed.

Similarly, the gate-source voltage Vgs101 of the transistor 101 can beset to the voltage Vsig+Vth−Vi1−Vα, which is a value unrelated to Vel.Consequently, variations in volt-ampere characteristic between thelight-emitting elements 104 a can be prevented from adversely affectingthe value of a current supplied to the light-emitting elements 104 a.Alternatively, even if the light-emitting element 104 a deteriorates andthe volt-ampere characteristic of the light-emitting element 104 a andhence Vel are changed, this change can be prevented from adverselyaffecting the value of a current supplied to the light-emitting element104 a. Therefore, high-quality images with less unevenness can bedisplayed.

Note that in part of the period in the fifth operation, the transistor101 can be forcibly turned off so that the light-emitting element 104 adoes not emit light. In other words, a non-lighting period can beprovided. For example, by turning on the switch 12, the transistor 101can be turned off.

For a semiconductor device according to one aspect of the presentinvention, the gate of the transistor 101 is held at the potential Vi2in the second operation. By the operation, even when the transistor 101is a normally on transistor, in other words, even when the thresholdvoltage Vthn is negative, electric charge accumulated in the capacitor102 until the potential of the source of the transistor 101 gets higherthan the potential Vi2 of the gate of the transistor 101 can bereleased. Accordingly, in a semiconductor device according to one aspectof the present invention, even when the transistor 101 is a normally ontransistor, in the fifth operation, the gate-source voltage Vgs 101 ofthe transistor 101 can be set to a value obtained by taking thethreshold voltage Vth of the transistor 101 into consideration.

FIGS. 7A to 7E are schematic views illustrating the circuit 100 in theperiods T11 to T15. For a semiconductor device according to one aspectof the present invention, the circuit 100 is set in states illustratedin FIGS. 7A to 7E in the respective periods. Therefore, a semiconductordevice according to one aspect of the present invention is not limitedto the circuits 100 with the structures illustrated in FIGS. 1A to 1D,FIGS. 2A to 2D, FIGS. 3A to 3D, and FIGS. 4A to 4D. For a semiconductordevice according to one aspect of the present invention, the placementor number of switches and the number of wirings supplying the potentialscan be changed as appropriate such that the circuit 100 is set in thestates illustrated in FIGS. 7A to 7E.

For a semiconductor device according to one aspect of the presentinvention, the circuit 100 in FIG. 1B may further include a capacitor105 connected to the load 104. Similarly, for a semiconductor deviceaccording to one aspect of the present invention, the circuit 100 inFIG. 1C may further include a capacitor 105 connected to thelight-emitting element 104 a. Similarly, for a semiconductor deviceaccording to one aspect of the present invention, the circuit 100 inFIG. 1D may further include a capacitor 105 connected to thelight-emitting element 104 b.

A semiconductor device in FIG. 8A corresponds to the circuit 100 in FIG.1B further including a capacitor 105 connected to the load 104.Specifically, one electrode of the capacitor 105 is connected to theother electrode of the capacitor 103 and one of a source and a drain ofthe transistor 101. The other electrode of the capacitor 105 isconnected to the wiring 26. Note that although FIG. 8A shows the casewhere the circuit 100 includes the load 104 as an example, thelight-emitting element 104 a or the light-emitting element 104 b may beused instead of the load 104 in FIG. 8A.

Note that the wiring 26 may be connected to a variety of wirings. Forexample, the wiring 26 may be connected to the wiring 22, the wiring 23,the wiring 24, a wiring in another circuit 100, a scan line, a gateline, a wiring connected to the gate of a transistor, or the like. Thus,the number of wirings can be reduced.

A semiconductor device in FIG. 8B corresponds to the circuit 100 in FIG.8A, in which the wiring 26 is connected to the wiring 24. Note thatalthough FIG. 8B shows the case where the circuit 100 includes the load104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.8B. When the wiring 26 is connected to the wiring 24, the number ofwirings 26 can be reduced.

A semiconductor device in FIG. 8C corresponds to the circuit 100 in FIG.8A, in which the wiring 26 is connected to the wiring 23. Note thatalthough FIG. 8C shows the case where the circuit 100 includes the load104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.8C. When the wiring 26 is connected to the wiring 23, the number ofwirings 26 can be reduced.

A semiconductor device in FIG. 8D corresponds to the circuit 100 in FIG.8A, in which the wiring 26 is connected to the wiring 22. Note thatalthough FIG. 8D shows the case where the circuit 100 includes the load104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.8D. When the wiring 26 is connected to the wiring 22, the number ofwirings 26 can be reduced.

When the capacitor 105 connected to the load 104, the light-emittingelement 104 a, or the light-emitting element 104 b is added to thecircuit 100, in the third operation and the fourth operation describedin this embodiment, fluctuations in electric charge at one of the sourceand the drain of the transistor 101 can be suppressed, so that thevoltage Vα can be reduced. Thus, the gate-source voltage Vgs can beclose to an ideal value, i.e., the voltage Vsig+Vth−Vi1, so that currentsupplied to the load 104, the light-emitting element 104 a, or thelight-emitting element 104 b can be set closer to a value that reflectsaccurately the voltage Vsig.

Alternatively, the capacitance of the capacitor 105 is adjusted asappropriate, so that the amount of change in potential due to theelectric charge Q in the period T14 can be adjusted. Thus, variations inmobility can be reduced more appropriately.

Note that the area of the electrodes of the capacitor 105 is preferablysmaller than the area of the electrodes of the load 104 (thelight-emitting element 104 a), more preferably ½ or less of the area ofthe electrodes of the load 104, still more preferably ⅓ or less of thearea of the electrodes of the load 104. Alternatively, the capacitanceof the capacitor 105 is preferably lower than the capacitance of theload 104 (the light-emitting element 104 a), more preferably ½ or less,still more preferably ⅓ or less of the capacitance of the load 104.Accordingly, optimum operation can be performed without changing thelayout area. Note that one aspect of the embodiment of the presentinvention is not limited thereto.

Note that the total area of the electrodes of the capacitor 105 and theelectrodes of the load 104 (the light-emitting element 104 a) ispreferably larger than the area of the electrodes of the capacitor 103,more preferably 2 or more times, still more preferably 5 or more timesthe area of the electrodes of the capacitor 103. Alternatively, thetotal capacitance of the capacitor 105 and the load 104 (thelight-emitting element 104 a) is preferably higher than the capacitanceof the capacitor 103, more preferably 2 or more times, still morepreferably 5 or more times the capacitance of the capacitor 103.Accordingly, when voltage is divided by the load 104 (the light-emittingelement 104 a) and each of the capacitors 103 and 105, higher voltagecan be applied to the capacitor 103. Note that one aspect of theembodiment of the present invention is not limited thereto.

Note that the area of the electrodes of the capacitor 105 is preferablysmaller than the area of the electrodes of the capacitor 102 or 103 (thelight-emitting element 104 a), more preferably ½ or less of the area ofthe electrodes of the capacitor 102 or 103, still more preferably ⅓ orless of the area of the electrodes of the capacitor 102 or 103.Alternatively, the capacitance of the capacitor 105 is preferably lowerthan the capacitance of the capacitor 102 or 103 (the light-emittingelement 104 a), more preferably ½ or less, still more preferably ⅓ orless of the capacitance of the capacitor 102 or 103. Accordingly,optimum operation can be performed without changing the layout area.Note that one aspect of the embodiment of the present invention is notlimited thereto.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 8A to 8D.

Semiconductor devices illustrated in FIGS. 9A to 9D include, in additionto the circuits 100 in FIGS. 8A to 8D, a circuit 201 having a functionof supplying a constant voltage or a signal to the wiring 21, a circuit202 having a function of supplying a constant voltage or a signal to thewiring 22, a circuit 203 having a function of supplying a constantvoltage or a signal to the wiring 23, and a circuit 204 having afunction of supplying a constant voltage or a signal to the wiring 24.The circuit 100 in FIG. 9A further includes a circuit 208 having afunction of supplying a constant voltage or a signal to the wiring 26.An example of the circuit 208 is a power supply circuit. Accordingly,the wiring 26 has a function of transmitting or supplying apredetermined potential. Alternatively, the wiring 26 functions as acapacitance wiring. Note that the potential of the wiring 26 ispreferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 26may vary like a pulse signal.

Any of the circuits 100 in FIGS. 1B to 1D and FIGS. 8B to 8D may be usedas a pixel of a display device. In the case where pixels for a pluralityof hues are provided in the display device, the transistors 101 ofpixels for different hues may differ in the ratio between the channelwidth and the channel length. Similarly, the capacitors 105 of pixelsfor different hues may differ in capacitance.

FIG. 10A illustrates the case where the circuit 100 in FIG. 1B is usedas a pixel of a display device. In FIG. 10A, a circuit 100(R)corresponds to a pixel for red (R); a circuit 100(G), a pixel for green(G); a circuit 100(B), a pixel for blue (B). In one aspect of thepresent invention, at least one of a transistor 101(R) in the circuit100(R), a transistor 101(G) in the circuit 100(G), and a transistor101(B) in the circuit 100B may differ from the others in the ratiobetween the channel width and the channel length. With the abovestructure, currents supplied to a load 104(R) in the circuit 100(R), aload 104(G) in the circuit 100(G), and a load 104(B) in the circuit100(B) can be set at different values. As an example, the ratio betweenthe channel width and the channel length of the transistor 101 in apixel for a second color is preferably 1.2 times or more, morepreferably 1.5 times or more the ratio between the channel width and thechannel length of the transistor 101 in a pixel for a first color. Inaddition, the ratio between the channel width and the channel length ofthe transistor 101 in a pixel for a third color is preferably 1.5 timesor more, more preferably two times or more the ratio between the channelwidth and the channel length of the transistor 101 in a pixel for thefirst color. Note that one aspect of the embodiment of the presentinvention is not limited thereto.

FIG. 10B illustrates the case where the circuit 100 in FIG. 8A is usedas a pixel of a display device. In FIG. 10B, as in FIG. 10A, at leastone of a transistor 101(R) in the circuit 100(R), a transistor 101(G) inthe circuit 100(G), and a transistor 101(B) in the circuit 100B maydiffer from the others in the ratio between the channel width and thechannel length. With the above structure, currents supplied to a load104(R) in the circuit 100(R), a load 104(G) in the circuit 100(G), aload 104(B) in the circuit 100(B) can be set at different values.

In FIG. 10B, at least one of the capacitor 105(R) in the circuit 100(R),the capacitor 105(G) in the circuit 100(G), and the capacitor 105(B) inthe circuit 100(B) may differ from the others in the capacitance. As anexample, the capacitance of the capacitor 105 in a pixel for the secondcolor is preferably 1.2 times or more, more preferably 1.5 times or morethe capacitance of the capacitor 105 in a pixel for the first color. Inaddition, the capacitance of the capacitor 105 in a pixel for the thirdcolor is preferably 1.5 times or more, more preferably two times or morethe capacitance of the capacitor 105 in a pixel for the first color.Note that one aspect of the embodiment of the present invention is notlimited thereto.

FIGS. 10A and 10B illustrate the case where the circuit 100(R) includesthe load 104(R), the circuit 100(G) includes the load 104(G), and thecircuit 100(B) includes the load 104(B); however, in FIG. 10A or 10B,the light-emitting element 104 a or 104 b of an appropriate hue may beused instead of the load 104(R), the load 104(G), or the load 104(B).

FIG. 10B illustrates the case where the circuit 100 in FIG. 8A is usedas a pixel of a display device; however, each of the circuits 100 inFIGS. 8B to 8D may be used as a pixel of a display device.

A circuit 100 in FIG. 11A is a semiconductor device according to oneaspect of the present invention. The circuit 100 includes a switch 11, aswitch 12, a switch 13, a switch 14, a transistor 101, a capacitor 102,and a capacitor 103. Note that FIG. 11A shows the case where thetransistor 101 is an n-channel transistor. The structure in FIG. 11Acorresponds to the structure in FIG. 1A, to which the switch 14 isadded. Therefore, the description for FIG. 1A may apply to FIG. 11A.

Specifically, in FIG. 11A, the switch 11 has a function of controllingthe conduction between a wiring 21 and one electrode of the capacitor102 or one electrode of the capacitor 103. The switch 12 has a functionof controlling the conduction between a wiring 22 and the otherelectrode of the capacitor 102 and between the wiring 22 and a gate ofthe transistor 101. The switch 13 has a function of controlling theconduction between one of a source and a drain of the transistor 101 orthe other electrode of the capacitor 103 and the one electrode of thecapacitor 102 or the one electrode of the capacitor 103. The switch 14has a function of controlling the conduction between the one of thesource and the drain of the transistor 101 or the other electrode of thecapacitor 103 and the wiring 25. The other of the source and the drainof the transistor 101 is connected to a wiring 23. The one of the sourceand the drain of the transistor 101 and the other electrode of thecapacitor 103 is connected to a wiring 24.

Note that the circuit 100 in FIG. 11A may include a load 104 asillustrated in FIG. 11B. In the circuit 100 in FIG. 11B, the load 104 isconnected between the one of the source and the drain of the transistor101 or the other electrode of the capacitor 103 and the wiring 24.

FIG. 11C illustrates the structure of a circuit 100 which uses alight-emitting element 104 a as the load 104. FIG. 11C illustrates thecase where an anode of the light-emitting element 104 a is connected tothe one of the source and the drain of the transistor 101 and the otherelectrode of the capacitor 103, and a cathode of the light-emittingelement 104 a is connected to the wiring 24.

FIG. 11D illustrates the structure of a circuit 100 which uses alight-emitting element 104 b as the load 104. FIG. 11D illustrates thecase where a cathode of the light-emitting element 104 b is connected tothe one of the source and the drain of the transistor 101 or the otherelectrode of the capacitor 103, and an anode of the light-emittingelement 104 b is connected to the wiring 24. Note that FIG. 11D showsthe case where the transistor 101 is a p-channel transistor.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 11A to 11D.

Semiconductor devices illustrated in FIGS. 12A to 12D include, inaddition to the circuits 100 in FIGS. 11A to 11D, a circuit 220 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 221 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 222 having a function of supplying aconstant voltage or a signal to the wiring 23, a circuit 223 having afunction of supplying a constant voltage or a signal to the wiring 24,and a circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25.

Specifically, the circuit 220 has a function of supplying a potentialVsig to the wiring 21. An example of the circuit 220 is a source driver(a signal line driver circuit). Accordingly, the wiring 21 has afunction of transmitting or supplying the potential Vsig. The wiring 21functions as a video signal line.

The circuit 221 has a function of supplying a potential Vi2 to thewiring 22. An example of the circuit 221 is a power supply circuit.Accordingly, the wiring 22 has a function of transmitting or supplyingthe potential Vi2. Alternatively, the wiring 22 functions as aninitialization line. Note that the potential of the wiring 22 ispreferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 22may vary like a pulse signal.

The circuit 222 has a function of supplying, for example, a supplyvoltage (a high supply potential or a low supply potential), e.g., apotential VDD or a potential VSS to the wiring 23. An example of thecircuit 222 is a power supply circuit. Accordingly, the wiring 23 has afunction of transmitting or supplying a power supply potential.Alternatively, the wiring 23 has a function of supplying current to thetransistor 101. Alternatively, the wiring 23 has a function of supplyingcurrent to the load 104. The wiring 23 functions as a power supply line.Alternatively, the wiring 23 functions as a current supply line. Notethat the potential of the wiring 23 is preferably constant; however, oneaspect of the embodiment of the present invention is not limitedthereto. The potential of the wiring 23 may vary like a pulse signal.For example, the potential of the wiring 23 may be a potential at whichnot only forward bias voltage but also reverse bias voltage is appliedto the load 104.

The circuit 223 has a function of supplying, for example, a supplyvoltage (a low supply potential or a high supply potential), e.g., apotential Vcat to the wiring 24. An example of the circuit 223 is apower supply circuit. Accordingly, the wiring 24 has a function oftransmitting or supplying a power supply potential. Alternatively, thewiring 24 has a function of supplying current to the load 104.Alternatively, the wiring 24 has a function of supplying current to thetransistor 101. The wiring 24 functions as a common line. Alternatively,the wiring 24 functions as a negative line. Alternatively, the wiring 24functions as a positive line. Note that the potential of the wiring 24is preferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 24may vary like a pulse signal. For example, the potential of the wiring24 may be a potential at which not only forward bias voltage but alsoreverse bias voltage is applied to the load 104.

The circuit 224 has a function of supplying a potential Vi1 to thewiring 25. An example of the circuit 224 is a power supply circuit.Accordingly, the wiring 25 has a function of transmitting or supplyingthe potential Vi1. Alternatively, the wiring 25 functions as aninitialization line. Note that the potential of the wiring 25 ispreferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 25may vary like a pulse signal.

Note that in FIGS. 12A to 12D, the semiconductor devices each include,in addition to the circuit 100, the circuits 220, 221, 222, 223, and224, as an example. However, a semiconductor device according to oneaspect of the present invention does not necessarily include all of thecircuits 220, 221, 222, 223, and 224, and may include only one or someof these circuits.

Note that in the circuits 100 in FIGS. 11A to 11D and FIGS. 12A to 12D,as an example, the switch 11, the switch 12, the switch 13, and theswitch 14 may be transistors.

FIGS. 13A to 13D illustrate the structures of circuits 100 whichcorrespond to the circuits 100 in FIGS. 11A to 11D each using atransistor 11 t as the switch 11, a transistor 12 t as the switch 12, atransistor 13 t as the switch 13, and a transistor 14 t as the switch14. FIGS. 13A to 13D illustrate the case where the transistor 11 t, thetransistor 12 t, the transistor 13 t, and the transistor 14 t are alln-channel transistors. When the transistor 11 t, the transistor 12 t,the transistor 13 t, and the transistor 14 t have the same polarity,these transistors can be manufactured in fewer steps. However, oneaspect of the embodiment of the present invention is not limited tothis; these transistors may have different polarities.

Note that in FIGS. 13A to 13D, a gate of the transistor 11 t isconnected to a wiring 31. The transistor 11 t is turned on or off inresponse to the potential supplied to the wiring 31. A gate of thetransistor 12 t is connected to a wiring 32. The transistor 12 t isturned on or off in response to the potential supplied to the wiring 32.A gate of the transistor 13 t is connected to a wiring 33. Thetransistor 13 t is turned on or off in response to the potentialsupplied to the wiring 33. A gate of the transistor 14 t is connected toa wiring 34. The transistor 14 t is turned on or off in response to thepotential supplied to the wiring 34. Therefore, it is preferable thatthe potentials of the wirings 31 to 34 be pulsed potentials and notconstant; however, one aspect of the embodiment of the present inventionis not limited to this. Alternatively, the wirings 31 to 34 eachfunction as a gate signal line, selection signal line, or a scan line.

Note that at least two of these wirings 31 to 34 can be connected toeach other. Alternatively, at least one of these wirings 31 to 34 can beconnected to at least one of the wirings 31 to 34 in another circuit100.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 13A to 13D.

Semiconductor devices illustrated in FIGS. 14A to 14D include, inaddition to the circuits 100 in FIGS. 13A to 13D, a circuit 230 having afunction of supplying a constant voltage or a signal to the wiring 31, acircuit 231 having a function of supplying a constant voltage or asignal to the wiring 32, a circuit 232 having a function of supplying aconstant voltage or a signal to the wiring 33, and a circuit 233 havinga function of supplying a constant voltage or a signal to the wiring 34.Examples of the circuits 230, 231, 232, and 233 include gate drivers(scan line driver circuits).

Note that in FIGS. 14C and 14D, the semiconductor devices each include,in addition to the circuit 100, the circuits 230, 231, 232, and 233, asan example. However, a semiconductor device according to one aspect ofthe present invention does not necessarily include all of the circuits230, 231, 232, and 233, and may include only one or some of thesecircuits.

Note that the circuits 220, 221, 222, 223, 224, 230, 231, 232, and 233may be the same circuit or different circuits.

FIG. 38A illustrates the structure of a circuit 100 corresponding to thecircuit 100 in FIG 13C, in which the transistors 101 and 14 t aren-channel transistors and the transistors 11 t, 12 t, and 13 t arep-channel transistors. FIG. 38B illustrates the structure of a circuit100 corresponding to the circuit 100 in FIG. 13D, in which thetransistors 101 and 14 t are p-channel transistors and the transistors11 t, 12 t, and 13 t are p-channel transistors.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 38A and 38B.

Semiconductor devices illustrated in FIGS. 38C and 38D include, inaddition to the circuits 100 in FIGS. 38A and 38B, a circuit 230 havinga function of supplying a constant voltage or a signal to the wiring 31,a circuit 231 having a function of supplying a constant voltage or asignal to the wiring 32, a circuit 232 having a function of supplying aconstant voltage or a signal to the wiring 33, and a circuit 233 havinga function of supplying a constant voltage or a signal to the wiring 34.

Note that in FIGS. 38C and 38D, the semiconductor devices each include,in addition to the circuit 100, the circuits 230, 231, 232, and 233, asan example. However, a semiconductor device according to one aspect ofthe present invention does not necessarily include all of the circuits230, 231, 232, and 233, and may include only one or some of thesecircuits.

In many cases, the transistor 101 operates in a saturation region whencurrent flows therethrough. Therefore, in FIGS. 13A to 13D, FIGS. 14A to14D, and FIGS. 38A to 38D, the transistor 101 preferably has a longerchannel length or gate length than the transistor 11 t, the transistor12 t, the transistor 13 t and/or the transistor 14 t. The channel lengthor gate length of the transistor 101 is preferably 5 or more times,further preferably 10 or more times that of the transistor 11 t, thetransistor 12 t, the transistor 13 t and/or the transistor 14 t. As anexample, the channel length or gate length of the transistor 101 ispreferably 10 μm or more, more preferably 20 μm or more. When thechannel length or the gate length is increased, characteristics in asaturation region have a flat slope; accordingly, a kink effect can bereduced. Alternatively, the channel width or gate width of thetransistor 101 is larger than that of the transistor 11 t, thetransistor 12 t, the transistor 13 t and/or the transistor 14 t, so thatmuch current flows through the transistor 101 even in a saturationregion. The channel width or the gate width of the transistor 101 ispreferably 5 or more times, further preferably 10 or more times that ofthe transistor 11 t, the transistor 12 t, the transistor 13 t and/or thetransistor 14 t. The channel width or the gate width of the transistor101 is preferably 20 μm or more, more preferably 30 μm or more. Notethat one aspect of an embodiment of the present invention is not limitedthereto.

In the semiconductor devices in FIGS. 13A to 13D and FIGS. 38A and 38B,both of the gates of the transistors 12 t and 13 t may be connected toone wiring. FIGS. 15A to 15D show semiconductor devices corresponding tothose in FIGS. 13A to 13D, in which the gates of the transistors 12 tand 13 t are connected to the wiring 32, as an example. The transistors12 t and 13 t are turned on or off in response to the potential suppliedto the wiring 32.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 15A to 15D.

Semiconductor devices illustrated in FIGS. 16A to 16D include, inaddition to the circuits 100 in FIGS. 15A to 15D, a circuit 230 having afunction of supplying a constant voltage or a signal to the wiring 31, acircuit 231 having a function of supplying a constant voltage or asignal to the wiring 32, and a circuit 233 having a function ofsupplying a constant voltage or a signal to the wiring 34.

Note that in FIGS. 16A to 16D, the semiconductor devices each include,in addition to the circuit 100, the circuits 230, 231, and 233, as anexample. However, a semiconductor device according to one aspect of thepresent invention does not necessarily include all of the circuits 230,231, and 233, and may include only one or some of these circuits.

FIG. 42A illustrates the structure of a circuit 100 corresponding to thecircuit 100 in FIG. 16C, in which the transistors 101, 11 t, and 14 tare n-channel transistors and the transistors 12 t and 13 t arep-channel transistors. FIG. 42B illustrates the structure of a circuit100 corresponding to the circuit 100 in FIG. 16D, in which thetransistors 101, 11 t, and 14 t are p-channel transistors and thetransistors 12 t and 13 t are n-channel transistors.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 42A and 42B.

Semiconductor devices illustrated in FIGS. 42C and 42D include, inaddition to the circuits 100 in FIGS. 42A and 42B, a circuit 230 havinga function of supplying a constant voltage or a signal to the wiring 31,a circuit 231 having a function of supplying a constant voltage or asignal to the wiring 32, and a circuit 233 having a function ofsupplying a constant voltage or a signal to the wiring 34. Examples ofthe circuits 230, 231, and 233 include gate drivers (scan line drivercircuits).

Note that in FIGS. 42C and 42D, the semiconductor devices each include,in addition to the circuit 100, the circuits 230, 231, and 233, as anexample. However, a semiconductor device according to one aspect of thepresent invention does not necessarily include all of the circuits 230,231, and 233, and may include only one or some of these circuits.

For the semiconductor devices illustrated in FIGS. 13A to 13D, inadjacent circuits 100, a gate of one transistor may be connected to agate of another transistor. For example, a gate of a transistor 11 t maybe connected to a gate of a transistor 14 t. FIG. 39 illustrates thecase where a gate of a transistor 11 t in a circuit 100(i, j) in thei-th column and the j-th row, and a gate of a transistor 14 t in acircuit 100(i, j+1) in the i-th column and the (j+1)th row are connectedto a wiring 31(j) in the j-th row.

For the semiconductor devices illustrated in FIGS. 15A to 15D, inadjacent circuits 100, a gate of one transistor may be connected to agate of another transistor. For example, a gate of a transistor 11 t maybe connected to a gate of a transistor 14 t. FIG. 40 illustrates thecase where a gate of a transistor 11 t in a circuit 100(i, j) in thei-th column and the j-th row, and a gate of a transistor 14 t in acircuit 100(i, j+1) in the i-th column and the (j+1)th row are connectedto a wiring 31(j) in the j-th row.

FIG. 39 and FIG. 40 each illustrate the case where a gate of atransistor 11 t in a circuit 100(i, j) in the i-th column and the j-throw, and a gate of a transistor 14 t in a circuit 100(i, j+1) in thei-th column and the (j+1)th row are connected to a wiring 31(j) in thej-th row. However, one aspect of the present invention is not limited tothis structure. For example, in the case of the semiconductor devicesillustrated in FIGS. 14A to 14D and FIGS. 38C and 38D, the circuit 230may supply a potential to a wiring 31(j) in the j-th row and to a wiring34(j+1) in the (j+1)th row.

FIG. 41 illustrates the case where the circuit 230 supplies a potentialto the wiring 31 and the wiring 34. Specifically, in FIG. 41 , apotential from a j-th output terminal out(j) of the circuit 230, issupplied to the wiring 31(j) in the j-th row and to the wiring 34(j+1)in the (j+1)th row. In other words, for example, wirings in differentrows are connected to each other between a scan line driver circuit anda pixel region.

The following describes the operation of one aspect of a semiconductordevice according to the present invention, taking a circuit 100illustrated in FIG. 11C as an example.

The operation of the circuit 100 illustrated in FIG. 11C can be mainlydivided into a first operation, a second operation, a third operation,and a fourth operation. Note that one aspect of an embodiment of thepresent invention is not limited thereto, and another operation can beadded or part of the operation can be omitted.

Note that the circuit in FIG. 11C corresponds to the circuit in FIG. 1Cto which a switch 14 is added; therefore, the third operation (theperiod T13) shown in FIG. 6A can be omitted.

FIG. 17A is an example of a timing chart showing the operations of theswitch 11, the switch 12, the switch 13, and the switch 14, thepotential of the wiring 21, and the gate-source voltage of thetransistor 101 (Vgs101) in the circuit 100 illustrated in FIG. 11C.

First, the first operation in the period T11 is described. In the periodT11, as illustrated in FIG. 17A, the switch 11 is off, and the switch12, the switch 13, and the switch 14 are on. Thus, in the period T11, asillustrated in FIG. 17B, the voltage Vi2−Vi1 is supplied to thecapacitor 102, the potential of the anode of the light-emitting element104 a becomes the potential Vi1, and the gate-source voltage Vgs101 ofthe transistor 101 becomes the voltage Vi2−Vi1. In other words, thetransistor 101 and the capacitor 102 are initialized.

Note that in the case where the potential of the wiring 21 does notcause an adverse effect, the switch 11 may be on. In that case, theswitch 14 may be off.

Note that the switch 13 may be off.

Next, the second operation in the period T12 is described. In the periodT12, as illustrated in FIG. 17A, the switch 11 and the switch 14 areoff, and the switch 12 and the switch 13 are on. When the switch 11 andthe switch 14 are off, electric charge accumulated in the capacitor 102is released through the transistor 101, and the potential of the sourceof the transistor 101 is raised. Then, when the transistor 101 is turnedoff, the release of the electric charge from the capacitor 102 isstopped. The threshold voltage Vth of the transistor 101 is eventuallyheld in the capacitor 102. Thus, in the period T12, as illustrated inFIG. 17C, the threshold voltage Vth is held in the capacitor 102, thepotential of the anode of the light-emitting element 104 a becomes thepotential Vi2−Vth, and the gate-source voltage Vgs101 of the transistor101 becomes the threshold voltage Vth. That is, the threshold voltageVth of the transistor 101 can be acquired.

Note that in some cases, it takes a very long time for Vgs101 to beequal to the threshold voltage Vth of the transistor 101. Accordingly,in many cases, an operation is performed while Vgs101 is not completelylowered to the threshold voltage Vth. That is, in many cases, the periodT12 is terminated while Vgs101 is slightly higher than the thresholdvoltage Vth. In other words, at the termination of the period T12,Vgs101 becomes voltage based on the threshold voltage.

Note that the second operation can be performed regardless of whetherthe threshold voltage Vth of the transistor 101 is positive voltage ornegative voltage. This is because the potential of the source of thetransistor 101 can be raised until the transistor 101 is turned off. Inother words, when the potential of the source of the transistor 101becomes higher than potential of the gate of the transistor 101, thetransistor 101 can be eventually turned off and Vgs101 can become Vth.Thus, the second operation can be performed without problems regardlessof whether the transistor 101 is an enhancement (normally-off)transistor or a depletion (normally-on) transistor.

Note that when the potential of the anode of the light-emitting element104 a becomes high, it is preferable that current does not flow to thelight-emitting element 104 a. For this purpose, the potential Vi2 ispreferably a low potential so that current does not flow to thelight-emitting element 104 a. Note that one aspect of an embodiment ofthe present invention is not limited thereto. If it is possible not tosupply current to the light-emitting element 104 a when a switchprovided in series with the light-emitting element 104 a is turned off,the potential Vi2 may be a high potential.

Next, the third operation in the period T13 is described. In the periodT13, as illustrated in FIG. 17A, the switch 11 and the switch 14 are on,and the switch 12 and the switch 13 are off. In addition, the potentialVsig is supplied to the wiring 21. Thus, in the period T13, asillustrated in FIG. 18A, the threshold voltage Vth (or the voltage basedon Vth) is held in the capacitor 102, a voltage Vsig−Vi1 is held in thecapacitor 103, the potential of the anode of the light-emitting element104 a becomes a potential Vi1, the potential of the gate of thetransistor 101 becomes the potential Vsig+Vth, and the gate-sourcevoltage Vgs101 of the transistor 101 becomes a voltage Vsig+Vth−Vi1.Thus, the potential Vsig can be supplied to the capacitor 103.Alternatively, the sum of the voltage across the capacitor 102 and thevoltage across the capacitor 103 can be equal to the gate-source voltageof the transistor 101.

Note that in that case, the switch 14 can be turned off.

The fourth operation in the period T14 is described. In the period T14,as illustrated in FIG. 17A, the switch 11, the switch 12, the switch 13,and the switch 14 are off. Thus, in the period T14, as illustrated inFIG. 18B, the threshold voltage Vth is held in the capacitor 102, thevoltage Vsig−Vi1 is held in the capacitor 103, the potential of theanode of the light-emitting element 104 a becomes the potential Vel, thepotential of the gate of the transistor 101 becomes the potentialVsig+Vth+Vel and the gate-source voltage Vgs101 of the transistor 101becomes the voltage Vsig+Vth−Vi1. Thus, current based on the potentialVsig can flow to the light-emitting element 104 a, so that thelight-emitting element 104 a can emit light at luminance based on thepotential Vsig.

In the fourth operation, the gate-source voltage Vgs101 of thetransistor 101 can be set to Vsig+Vth−Vi1 by taking the thresholdvoltage Vth of the transistor 101 into consideration. Consequently,variations in threshold voltage Vth between the transistors 101 can beprevented from adversely affecting the value of a current supplied tothe light-emitting elements 104 a. Alternatively, even if the transistor101 deteriorates and the threshold voltage Vth is changed, the change inthe threshold voltage Vth can be prevented from adversely affecting thevalue of a current supplied to the light-emitting element 104 a.Therefore, high-quality images with less unevenness can be displayed.

Similarly, the gate-source voltage Vgs101 of the transistor 101 can beset to the voltage Vsig+Vth−Vi1, which is a value unrelated to Vel.Consequently, variations in volt-ampere characteristic between thelight-emitting elements 104 a can be prevented from adversely affectingthe value of a current supplied to the light-emitting elements 104 a.Alternatively, even if the light-emitting element 104 a deteriorates andthe volt-ampere characteristic of the light-emitting element 104 a andhence Vel are changed, this change can be prevented from adverselyaffecting the value of a current supplied to the light-emitting element104 a. Therefore, high-quality images with less unevenness can bedisplayed.

Note that in part of the period in the fourth operation, the transistor101 can be forcibly turned off or current cannot be supplied to thelight-emitting element 104 a so that the light-emitting element 104 adoes not emit light. In other words, a non-lighting period can beprovided. For example, by turning on the switch 12, the transistor 101can be turned off. Alternatively, by turning on the switch 14, currentcannot be supplied to the light-emitting element 104 a.

For a semiconductor device according to one aspect of the presentinvention, the gate of the transistor 101 is held at the potential Vi2in the second operation. By the operation, even when the transistor 101is a normally on transistor, in other words, even when the thresholdvoltage Vthn is negative, electric charge accumulated in the capacitor102 until the potential of the source of the transistor 101 gets higherthan the potential Vi2 of the gate of the transistor 101 can bereleased. Accordingly, in a semiconductor device according to one aspectof the present invention, even when the transistor 101 is a normally ontransistor, in the fourth operation, the gate-source voltage Vgs 101 ofthe transistor 101 can be set to a value obtained by taking thethreshold voltage Vth of the transistor 101 into consideration.

The capacitance of the capacitor 103 is preferably lower than theparasitic capacitance of the load 104 (the light-emitting element 104a), more preferably ½ or less, still more preferably ⅕ or less of thecapacitance of the load 104. Alternatively, the area of the electrodesof the capacitor 103 is preferably smaller than the area of theelectrodes of the load 104 (the light-emitting element 104 a), morepreferably ½ or less of the area of the electrodes of the load 104,still more preferably ⅕ or less of the area of the electrodes of theload 104. Note that one aspect of an embodiment of the present inventionis not limited thereto.

Note that the capacitance of the capacitor 102 is preferably higher thanthe parasitic capacitance of the gate of the transistor 101, morepreferably 2 or more times the parasitic capacitance of the gate of thetransistor 101, still more preferably 5 or more times the parasiticcapacitance of the gate of the transistor 101. Alternatively, the areaof the electrodes of the capacitor 102 is preferably larger than thearea of a channel region of the transistor 101, more preferably 2 ormore times the area of the channel region of the transistor 101, stillmore preferably 5 or more times the area of the channel region of thetransistor 101. Alternatively, the area of the electrodes of thecapacitor 102 is preferably larger than the area of the gate electrodeof the transistor 101, more preferably 2 or more times the area of thegate electrode of the transistor 101, still more preferably 5 or moretimes the area of the gate electrode of the transistor 101. Accordingly,when the potential Vsig is input and voltage is divided by the capacitor102 and the gate capacitance of the transistor, a decrease in voltage ofthe capacitor 102 can be reduced. Note that one aspect of the embodimentof the present invention is not limited thereto.

Note that the capacitance of the capacitor 102 is preferably equal to orhigher than the capacitance of the capacitor 103. The difference betweenthe capacitance of the capacitor 102 and the capacitance of thecapacitor 103 is preferably ±20% or lower, more preferably ±10% orlower. Alternatively, the area of the electrodes of the capacitor 102 ispreferably equal to or larger than the area of the electrodes of thecapacitor 103. Accordingly, the semiconductor device can perform optimumoperation without changing the layout area. Note that one aspect of theembodiment of the present invention is not limited thereto.

FIGS. 19A to 19D are schematic views illustrating the circuit 100 in theperiods T11 to T14. For a semiconductor device according to one aspectof the present invention, the circuit 100 is set in states illustratedin FIGS. 19A to 19D in the respective periods. Therefore, asemiconductor device according to one aspect of the present invention isnot limited to the circuits 100 with the structures illustrated in FIGS.11A to 11D, FIGS. 12A to 12D, FIGS. 13A to 13D, FIGS. 14A to 14D, FIGS.15A to 15D, and FIGS. 16A to 16D. For a semiconductor device accordingto one aspect of the present invention, the placement or number ofswitches and the number of wirings supplying the potentials can bechanged as appropriate such that the circuit 100 is set in the statesillustrated in FIGS. 19A to 19D.

Note that the period T16 during which the sixth operation is performedmay be provided after the period T13 during which the third operation isperformed and before the period T14 during which the fourth operation isperformed.

FIG. 20A is an example of a timing chart including the period T16,showing the operations of the switch 11, the switch 12, the switch 13,and the switch 14, the potential of the wiring 21, and the gate-sourcevoltage of the transistor 101 (Vgs101) in the circuit 100 illustrated inFIG. 11C.

The timing chart in FIG. 20A differs from the timing chart in FIG. 17Ain that the period T16 is provided between the period T13 and the periodT14.

The sixth operation in the period T16 is described. In the period T16,as illustrated in FIG. 20A, the switch 12 is on, and the switch 11, theswitch 13, and the switch 14 are off. Thus, in the period T16, asillustrated in FIG. 20B, the gate-source voltage Vgs101 of thetransistor 101 becomes the voltage Vsig+Vth−Vi1−Vα.

The potential Vα in the sixth operation fluctuates when the anode of thelight-emitting element 104 a is electrically floating (in a floatingstate). The value of the potential Vα depends on the ratio between thecapacitance of the light-emitting element 104 a and the capacitance ofthe capacitor 102 and capacitor 103 if the transistor 101 is off.However, the transistor 101 might be turned on depending on the value ofthe potential Vsig, and electric charge flows into the anode of thelight-emitting element 104 a through the transistor 101. Thus, the valueof the potential Vα depends on not only the capacitance ratio but alsoelectric charge flowing into the anode of the light-emitting element 104a.

The electric charge Q may produce the effect of suppressing suchvariations in mobility. The reason will be described below.

The electric charge Q is the amount of electric charge which flows fromthe drain to the source of the transistor 101 in the period T16.Accordingly, the electric charge Q is increased as the mobility of thetransistor 101 increases. As the electric charge Q is increased, thegate-source voltage Vgs 101 of the transistor 101 at the time when thelight-emitting element 104 a emits light is reduced. In other words, asthe mobility of the transistor 101 increases, correction is made by theelectric charge Q so that the value of current supplied to thelight-emitting element 104 a may be small, whereas, as the mobility ofthe transistor 101 decreases, correction is made by the electric chargeQ so that the value of current supplied to the light-emitting element104 a may not be so small. Thus, variations in mobility can besuppressed by the electric charge Q.

In the period 14 after the period T16, the gate-source voltage Vgs101 ofthe transistor 101 becomes the voltage Vsig+Vth−Vi1−Vα. Thus, for thetransistor 101, the gate-source voltage can be set to a value by takingthe threshold voltage Vth and the mobility into consideration.

For a semiconductor device according to one aspect of the presentinvention, as in FIGS. 8A to 8D, the circuit 100 in FIG. 11B may furtherinclude a capacitor 105 connected to the load 104. Similarly, for asemiconductor device according to one aspect of the present invention,the circuit 100 in FIG. 11C may further include a capacitor 105connected to the light-emitting element 104 a. Similarly, for asemiconductor device according to one aspect of the present invention,the circuit 100 in FIG. 11D may further include a capacitor 105connected to the light-emitting element 104 b.

A semiconductor device in FIG. 21A corresponds to the circuit 100 inFIG. 11B further including a capacitor 105 connected to the load 104.Specifically, one electrode of the capacitor 105 is connected to theother electrode of the capacitor 103 and the one of a source and a drainof the transistor 101. The other electrode of the capacitor 105 isconnected to the wiring 26. Note that although FIG. 21A shows the casewhere the circuit 100 includes the load 104 as an example, thelight-emitting element 104 a or the light-emitting element 104 b may beused instead of the load 104 in FIG. 21A.

Note that the wiring 26 may be connected to a variety of wirings. Forexample, the wiring 26 may be connected to the wiring 22, the wiring 23,the wiring 24, the wiring 25, a wiring in another circuit 100, a scanline, a gate line, a wiring connected to the gate of a transistor, orthe like. Thus, the number of wirings can be reduced.

A semiconductor device in FIG. 21B corresponds to the circuit 100 inFIG. 21A, in which the wiring 26 is connected to the wiring 24. Notethat although FIG. 21B shows the case where the circuit 100 includes theload 104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.21B. When the wiring 26 is connected to the wiring 24, the number ofwirings 26 can be reduced.

A semiconductor device in FIG. 21C corresponds to the circuit 100 inFIG. 21A, in which the wiring 26 is connected to the wiring 23. Notethat although FIG. 21C shows the case where the circuit 100 includes theload 104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.21C. When the wiring 26 is connected to the wiring 23, the number ofwirings 26 can be reduced.

A semiconductor device in FIG. 21D corresponds to the circuit 100 inFIG. 21A, in which the wiring 26 is connected to the wiring 22. Notethat although FIG. 21D shows the case where the circuit 100 includes theload 104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.21D. When the wiring 26 is connected to the wiring 22, the number ofwirings 26 can be reduced.

A semiconductor device in FIG. 21E corresponds to the circuit 100 inFIG. 21A, in which the wiring 26 is connected to the wiring 25. Notethat although FIG. 21E shows the case where the circuit 100 includes theload 104 as an example, the light-emitting element 104 a or thelight-emitting element 104 b may be used instead of the load 104 in FIG.21E. When the wiring 26 is connected to the wiring 25, the number ofwirings 26 can be reduced.

When the capacitor 105 connected in parallel to the load 104, thelight-emitting element 104 a or the light-emitting element 104 b isadded to the circuit 100, in the sixth operation and the fourthoperation described in the above embodiment, fluctuations in electriccharge at the one of the source and the drain of the transistor 101 canbe suppressed, so that a voltage Vα can be reduced. Thus, thegate-source voltage Vgs can be close to an ideal value, i.e., thevoltage Vsig+Vth−Vi1, so that current supplied to the load 104, thelight-emitting element 104 a, or the light-emitting element 104 b can beset to a value closer to that reflects accurately the voltage Vsig.

Alternatively, the capacitance of the capacitor 105 is adjusted asappropriate, so that the amount of change in potential due to theelectric charge Q in the period T16 can be adjusted. Thus, variations inmobility can be reduced more appropriately.

Note that the area of the electrodes of the capacitor 105 is preferablysmaller than the area of the electrodes of the load 104 (thelight-emitting element 104 a), more preferably ½ or less of the area ofthe electrodes of the load 104, still more preferably ⅓ or less of thearea of the electrodes of the load 104. Alternatively, the capacitanceof the capacitor 105 is preferably lower than the capacitance of theload 104 (the light-emitting element 104 a), more preferably ½ or less,still more preferably ⅓ or less of the capacitance of the load 104.Accordingly, optimum operation can be performed without changing thelayout area. Note that one aspect of the embodiment of the presentinvention is not limited thereto.

Note that the total area of the electrodes of the capacitor 105 and theelectrodes of the load 104 (the light-emitting element 104 a) ispreferably larger than the area of the electrodes of the capacitor 103,more preferably 2 or more times, still more preferably 5 or more timesthe area of the electrodes of the capacitor 103. Alternatively, thetotal capacitance of the capacitor 105 and the load 104 (thelight-emitting element 104 a) is preferably higher than the capacitanceof the capacitor 103, more preferably 2 or more times, still morepreferably 5 or more times the capacitance of the capacitor 103.Accordingly, when voltage is divided by the load 104 (the light-emittingelement 104 a) and each of the capacitors 103 and 105, higher voltagecan be applied to the capacitor 103. Note that one aspect of theembodiment of the present invention is not limited thereto.

Note that the area of the electrodes of the capacitor 105 is preferablysmaller than the area of the electrodes of the capacitor 102 or 103 (thelight-emitting element 104 a), more preferably ½ or less of the area ofthe electrodes of the capacitor 102 or 103, still more preferably ⅓ orless of the area of the electrodes of the capacitor 102 or 103.Alternatively, the capacitance of the capacitor 105 is preferably lowerthan the capacitance of the capacitor 102 or 103 (the light-emittingelement 104 a), more preferably ½ or less, still more preferably ⅓ orless of the capacitance of the capacitor 102 or 103. Accordingly,optimum operation can be performed without changing the layout area.Note that one aspect of the embodiment of the present invention is notlimited thereto.

Note that the wiring 25 may be connected to a variety of wirings. Forexample, the wiring 25 may be connected to the wiring 22, the wiring 24,the wiring 26, a wiring in another circuit 100, a scan line, a gateline, a wiring connected to the gate of a transistor, or the like. Thus,the number of wirings can be reduced.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 21A to 21D.

Semiconductor devices illustrated in FIGS. 22A to 22D include, inaddition to the circuits 100 in FIGS. 21A to 21D, a circuit 220 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 221 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 222 having a function of supplying aconstant voltage or a signal to the wiring 23, a circuit 223 having afunction of supplying a constant voltage or a signal to the wiring 24,and a circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25. The circuit 100 in FIG. 22A further includes acircuit 225 having a function of supplying a fixed voltage or signal tothe wiring 26.

Any of the circuits 100 in FIGS. 11B to 11D and FIGS. 21B to 21D may beused as a pixel of a display device. In the case where pixels for aplurality of hues are provided in the display device, the transistors101 of pixels for different hues may differ in the ratio between thechannel width and the channel length. Similarly, the capacitors 105 ofpixels for different hues may differ in capacitance.

FIG. 23A illustrates the case where the circuit 100 in FIG. 11B is usedas a pixel of a display device. In FIG. 23A, a circuit 100(R)corresponds to a pixel for red (R); a circuit 100(G), a pixel for green(G); a circuit 100(B), a pixel for blue (B). In one aspect of thepresent invention, at least one of a transistor 101(R) in the circuit100(R), a transistor 101(G) in the circuit 100(G), and a transistor101(B) in the circuit 100B may differ from the others in the ratiobetween the channel width and the channel length. With the abovestructure, currents supplied to a load 104(R) in the circuit 100(R), aload 104(G) in the circuit 100(G), a load 104(B) in the circuit 100(B)can be set at different values.

FIG. 23B illustrates the case where the circuit 100 in FIG. 21A is usedas a pixel of a display device, as an example. In FIG. 23B, as in FIG.23A, at least one of a transistor 101(R) in the circuit 100(R), atransistor 101(G) in the circuit 100(G), and a transistor 101(B) in thecircuit 100B may differ from the others in the ratio between the channelwidth and the channel length. With the above structure, currentssupplied to a load 104(R) in the circuit 100(R), a load 104(G) in thecircuit 100(G), a load 104(B) in the circuit 100(B) can be set atdifferent values.

In FIG. 23B, at least one of the capacitor 105(R) in the circuit 100(R),the capacitor 105(G) in the circuit 100(G), and the capacitor 105(B) inthe circuit 100(B) may differ from the others in the capacitance.

FIGS. 23A and 23B illustrate the case where the circuit 100(R) includesthe load 104(R), the circuit 100(G) includes the load 104(G), and thecircuit 100(B) includes the load 104(B); however, in FIG. 23A or 23B,the light-emitting element 104 a or 104 b of an appropriate hue may beused instead of the load 104(R), the load 104(G), or the load 104(B).

FIG. 23B illustrates the case where the circuit 100 in FIG. 21A is usedas a pixel of a display device; however, each of the circuits 100 inFIGS. 21B to 21E may be used as a pixel of a display device.

Note that variation in the threshold voltage or the like of thetransistor 101 is corrected in this embodiment, but one aspect of anembodiment of the present invention is not limited thereto. For example,the circuit can operate to supply current to the load 104 without theoperation for correcting variations in threshold voltage.

In this embodiment, an example of a basic principle is described. Thus,part or the whole of this embodiment can be freely combined with,applied to, or replaced with part or the whole of another embodiment.

Embodiment 2

In this embodiment, structure examples of the circuit 100 that is asemiconductor device according to one aspect of the present inventionare described. In this embodiment, a switch is added to the circuitdescribed in Embodiment 1 or the method for driving the circuitdescribed in Embodiment 1 is partly changed. Thus, the contentsdescribed in Embodiment 1 can also be applied to this embodiment.

FIGS. 24A to 24D each illustrate a structure example of a circuit 100.The circuits 100 in FIGS. 24A to 24D correspond respectively to thecircuits 100 in FIGS. 1A to 1D to which a switch 914 is added. Theswitch 914 has a function of controlling conduction between the other ofthe source and the drain of a transistor 101 and the wiring 23.Alternatively, the switch 914 has a function of controlling conductionbetween the wiring 23 and the wiring 24. Alternatively, the switch 914has a function of preventing current from flowing into the capacitor103. Alternatively, the switch 914 has a function of preventing currentfrom flowing into the capacitor 102. Alternatively, the switch 914 has afunction of preventing current from flowing into the load 104.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 24A to 24D.

Semiconductor devices illustrated in FIGS. 25A to 25D include, inaddition to the circuits 100 in FIGS. 24A to 24D, a circuit 201 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 202 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 203 having a function of supplying aconstant voltage or a signal to the wiring 23, and a circuit 204 havinga function of supplying a constant voltage or a signal to the wiring 24.

Note that in the circuits 100 in FIGS. 24A to 24D and FIGS. 25A to 25D,as an example, the switch 11, the switch 12, the switch 13, and theswitch 914 may be transistors.

Note that for example, as illustrated in FIG. 86 , when the switch 914is a transistor 914 t, a gate of the transistor 914 t may be connectedto a wiring 932, and the wiring 932 may be connected to a circuit 9206having a function of supplying a constant voltage or a signal. Anexample of the circuit 9206 is a gate driver (a scan line drivercircuit).

Note that at least two of the wirings 31 to 33 and the wiring 932 can beconnected to each other. Alternatively, at least one of the wirings 31to 33 and the wiring 932 can be connected to at least one of the wirings31 to 33 and the wiring 932 in another circuit 100.

The circuits 100 in FIGS. 24A to 24D and FIGS. 25A and 25D can operatein the same way as the circuits 100 in FIGS. 1A to 1D and FIGS. 2A to2D. Note that, as an example, in the circuits 100 in FIGS. 24A to 24Dand FIGS. 25A and 25D, the switch 914 is preferably on in the periodsT11 to T13 and the period T15, and off in the period T14 (these periodsare described with reference to FIGS. 5A to 5C and FIGS. 6A to 6C).Thus, in the period T14, electric charge can be prevented from leakinginto the light-emitting element 104 a and the like through thetransistor 101. Note that one aspect of the embodiment of the presentinvention is not limited thereto.

Alternatively, in the period T13, the switch 914 may be off. In thatcase, current does not flow into the transistor 101, facilitatingcontrol of the potentials of the nodes in the circuit 100, such as thegate and the source of the transistor 101.

Alternatively, in the period T11, the switch 914 may be off. In thatcase, current does not flow into the transistor 101, facilitatingcontrol of the potentials of the nodes in the circuit 100, such as thegate and the source of the transistor 101.

Alternatively, the switch 914 is also turned off in a part of the periodT15. This prevents current from flowing into the light-emitting element104 a and the like, thereby providing a non-lighting period.

Note that the circuits 100 in FIG. 24B and FIG. 25B may each furtherinclude a capacitor 105 connected to the load 104 as in FIGS. 8A to 8D,FIGS. 9A to 9D, and FIG. 10B. Similarly, the circuits 100 in FIG. 24Cand FIG. 25C may each further include a capacitor 105 connected to thelight-emitting element 104 a. Similarly, the circuits 100 in FIG. 24Dand FIG. 25D may each further include a capacitor 105 connected to thelight-emitting element 104 b. Specifically, one electrode of thecapacitor 105 is connected to the other electrode of the capacitor 103and the one of a source and a drain of the transistor 101. The otherelectrode of the capacitor 105 is connected to a wiring 26, 24, 23, or22 which is additionally provided.

Any of the circuits 100 in FIGS. 24B to 24D and the circuitscorresponding to the circuits 100 in FIGS. 25B to 25D, to which thecapacitors 105 are added may be used as a pixel of a display device. Inthe case where pixels for a plurality of hues are provided in thedisplay device, the transistors 101 of pixels for different hues maydiffer in the ratio between the channel width and the channel length.

Note that the position of the switch 914 may be different from theposition in FIGS. 24A to 24D and FIGS. 25A to 25D. Specifically, forexample, the switch 914 can be provided at the position at which theswitch 914 can control conduction between the wiring 23 and the wiring24. FIGS. 26A to 26D each illustrate a structure example of a circuit100, as an example. The circuits 100 in FIGS. 26A to 26D correspondrespectively to the circuits 100 in FIGS. 1A to 1D to which a switch 914is added. The switch 914 has a function of controlling conductionbetween the one of a source and a drain of the transistor 101 and theother electrode of the capacitor 103. In addition, when the switch 13 ison, the switch 914 has a function of controlling conduction between oneof the source and the drain of the transistor 101 and the one electrodeof the capacitor 102 and conduction between one of a source and a drainof the transistor 101 and the one electrode of the capacitor 103.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 26A to 26D.

Semiconductor devices illustrated in FIGS. 27A to 27D include, inaddition to the circuits 100 in FIGS. 26A to 26D, a circuit 201 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 202 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 203 having a function of supplying aconstant voltage or a signal to the wiring 23, and a circuit 204 havinga function of supplying a constant voltage or a signal to the wiring 24.

Note that in the circuits 100 in FIGS. 26A to 26D and FIGS. 27A to 27D,as an example, the switch 11, the switch 12, the switch 13, and theswitch 914 may be transistors.

The circuits 100 in FIGS. 26A to 26D and FIGS. 27A to 27D can operate inthe same way as the circuits 100 in FIGS. 1A to 1D, FIGS. 2A to 2D,FIGS. 24A to 24D, or FIGS. 25A to 25D.

Note that the circuits 100 in FIG. 26B and FIG. 27B may each furtherinclude a capacitor 105 connected to the load 104 as in FIGS. 8A to 8D,FIGS. 9A to 9D, and FIG. 10B. Similarly, the circuits 100 in FIG. 26Cand FIG. 26C may each further include a capacitor 105 connected to thelight-emitting element 104 a. Similarly, the circuits 100 in FIG. 26Dand FIG. 27D may each further include a capacitor 105 connected to thelight-emitting element 104 b. Specifically, one electrode of thecapacitor 105 is connected to the other electrode of the capacitor 103.Further, the switch 914 controls conduction between the one electrode ofthe capacitor 105 and the one of the source and the drain of thetransistor 101. The other electrode of the capacitor 105 is connected toa wiring 26, 24, 23, or 22 which is additionally provided.

Any of the circuits 100 in FIGS. 26B to 26D and the circuitscorresponding to the circuits 100 in FIGS. 27B to 27D to which thecapacitor 105 is added may be used as a pixel of a display device. Inthe case where pixels for a plurality of hues are provided in thedisplay device, the transistors 101 of pixels for different hues maydiffer in the ratio between the channel width and the channel length.

Note that the position of the switch 914 may be different from theposition in FIGS. 24A to 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, andFIGS. 27A to 27D. FIGS. 28A to 28D each illustrate a structure exampleof a circuit 100, as an example. The circuits 100 in FIGS. 28A to 28Dcorrespond respectively to the circuits 100 in FIGS. 1A to 1D to which aswitch 914 is added. The switch 914 has a function of controllingconduction between the one of a source and a drain of a transistor 101and the other electrode of the capacitor 103. In addition, when theswitch 13 is on, the switch 914 has a function of controlling conductionbetween the one electrode of the capacitor 103 and the other electrodeof the capacitor 103, and conduction between the one electrode of thecapacitor 102 and the other electrode of the capacitor 103.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 28A to 28D.

Semiconductor devices illustrated in FIGS. 29A to 29D include, inaddition to the circuits 100 in FIGS. 28A to 28D, a circuit 201 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 202 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 203 having a function of supplying aconstant voltage or a signal to the wiring 23, and a circuit 204 havinga function of supplying a constant voltage or a signal to the wiring 24.

Note that in the circuits 100 in FIGS. 28A to 28D and FIGS. 29A to 29D,as an example, the switch 11, the switch 12, the switch 13, and theswitch 914 may be transistors.

The circuits 100 in FIGS. 28A to 28D and FIGS. 29A to 29D can operate inthe same way as the circuits 100 in FIGS. 1A to 1D, FIGS. 2A to 2D,FIGS. 24A to 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, and FIGS. 27A to27D. Note that, as an example, in the circuits 100 in FIGS. 28A to 28Dand FIGS. 29A to 29D, the switch 914 is preferably on in the periods T11to T13 and the period T15 shown in FIGS. 5A to 5C and FIGS. 6A to 6C,and off in the period T14. Thus, in the period T14, electric charge canbe prevented from leaking into the light-emitting element 104 a and thelike through the transistor 101. Note that one aspect of the embodimentof the present invention is not limited thereto.

Alternatively, in the period T11, the switch 914 may be off. Thus,current does not flow into the transistor 101, facilitating control ofpotential.

Alternatively, the switch 914 is also turned off in a part of the periodT15. This prevents current from flowing into the light-emitting element104 a and the like, thereby providing a non-lighting period.

In the period T12, the switch 914 may be turned off. Turing off theswitch 914 in the period T12 makes it possible to keep the anode of thelight-emitting element 104 a at the potential Vi1 in the period T12.Therefore, the fourth operation in the period T14 can be performed aftertermination of the second operation in the period T12 without the periodT13, that is, without the third operation.

Note that the circuits 100 in FIG. 28B and FIG. 29B may each furtherinclude a capacitor 105 connected to the load 104 as in FIGS. 8A to 8D,FIGS. 9A to 9D, FIG. 10B, and the like. Similarly, the circuits 100 inFIG. 28C and FIG. 29C may each further include a capacitor 105 connectedto the light-emitting element 104 a. Similarly, the circuits 100 in FIG.28D and FIG. 29D may each further include a capacitor 105 connected tothe light-emitting element 104 b. Specifically, one electrode of thecapacitor 105 is connected to the other electrode of the capacitor 103.Further, the switch 914 controls conduction between the one electrode ofthe capacitor 105 and the one of a source and a drain of the transistor101. The other electrode of the capacitor 105 is connected to a wiring26, 24, 23, or 22 which is additionally provided.

Any of the circuits 100 in FIGS. 28B to 28D and the circuitscorresponding to the circuits 100 in FIGS. 29B to 29D to which thecapacitor 105 is added may be used as a pixel of a display device. Inthe case where pixels for a plurality of hues are provided in thedisplay device, the transistors 101 of pixels for different hues maydiffer in the ratio between the channel width and the channel length.

Note that the position of the switch 914 may be different from that inFIGS. 24A to 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, FIGS. 27A to 27D,FIGS. 28A to 28D, and FIGS. 29A to 29D. FIGS. 30A to 30D each illustratea structure example of a circuit 100, as an example. The circuits 100 inFIGS. 30A to 30D correspond respectively to the circuits 100 in FIGS. 1Ato 1D to which a switch 914 is added. In FIG. 30A, the switch 914 has afunction of controlling conduction between the one of a source and adrain of a transistor 101 and the wiring 24, and conduction between theother electrode of the capacitor 103 and the wiring 24. In FIG. 30B, theswitch 914 has a function of controlling conduction between the one of asource and a drain of a transistor 101 and a load 104, and conductionbetween the other electrode of the capacitor 103 and the load 104. InFIG. 30C, the switch 914 has a function of controlling conductionbetween the one of a source and a drain of a transistor 101 and an anodeof a light-emitting element 104 a, and conduction between the otherelectrode of the capacitor 103 and the anode of the light-emittingelement 104 a. In FIG. 30D, the switch 914 has a function of controllingconduction between the one of a source and a drain of a transistor 101and a cathode of a light-emitting element 104 b, and conduction betweenthe other electrode of the capacitor 103 the cathode of thelight-emitting element 104 b.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 30A to 30D.

Semiconductor devices illustrated in FIGS. 31A to 31D include, inaddition to the circuits 100 in FIGS. 30A to 30D, a circuit 201 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 202 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 203 having a function of supplying aconstant voltage or a signal to the wiring 23, and a circuit 204 havinga function of supplying a constant voltage or a signal to the wiring 24.

Note that in the circuits 100 in FIGS. 30A to 30D and FIGS. 31A to 31D,as an example, the switch 11, the switch 12, the switch 13, and theswitch 914 may be transistors.

The circuits 100 in FIGS. 30A to 30D and FIGS. 31A to 31D can operate ina similar way to the circuits 100 in FIGS. 1A to 1D, FIGS. 2A to 2D,FIGS. 24A to 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, FIGS. 27A to 27D,FIGS. 28A to 28D, and FIGS. 29A to 29D. Note that, as an example, in thecircuits 100 in FIGS. 30A to 30D and FIGS. 31A to 31D, the switch 914 ispreferably on in the period T11 and the periods T13 to T15 shown inFIGS. 5A to 5C and FIGS. 6A to 6C, and off in the period T12. Note thatone aspect of the embodiment of the present invention is not limitedthereto. Therefore, turning off the switch 914 in the period T12 makesit possible to keep the anode of the light-emitting element 104 a at thepotential Vi1 in the period T12. Therefore, the fourth operation in theperiod T14 can be performed after termination of the second operation inthe period T12 without the period T13, that is, without the thirdoperation. Note that one aspect of the embodiment of the presentinvention is not limited thereto.

Alternatively, in the period T11, the switch 914 may be off. Thus,current does not flow into the light-emitting element 104 a and thelike, so that the potential Vi2 of the wiring 22 may be at a high value.

Alternatively, in the period T12, the switch 914 may be off. Thus,current does not flow into the light-emitting element 104 a and thelike, so that the potential Vi2 of the wiring 22 may be at a high value.

Alternatively, the switch 914 is also turned off in a part of the periodT15. This prevents current from flowing into the light-emitting element104 a and the like, thereby providing a non-lighting period.

Note that the circuits 100 in FIG. 30B and FIG. 31B may each furtherinclude a capacitor 105 connected to the load 104 as in FIGS. 8A to 8D,FIGS. 9A to 9D, FIG. 10B, FIGS. 21A to 21E, and FIGS. 22A to 22E.Similarly, the circuits 100 in FIG. 30C and FIG. 31C may each furtherinclude a capacitor 105 connected to the light-emitting element 104 a.Similarly, the circuits 100 in FIG. 30D and FIG. 31D may each furtherinclude a capacitor 105 connected to the light-emitting element 104 b.Specifically, one electrode of the capacitor 105 is connected to theother electrode of the capacitor 103 and the one of a source and a drainof the transistor 101. The other electrode of the capacitor 105 isconnected to a wiring 26, 24, 23, or 22 which is additionally provided.

Any of the circuits 100 in FIGS. 30B to 30D and the circuitscorresponding to the circuits 100 in FIGS. 31B to 31D to which thecapacitor 105 is added may be used as a pixel of a display device. Inthe case where pixels for a plurality of hues are provided in thedisplay device, the transistors 101 of pixels for different hues maydiffer in the ratio between the channel width and the channel length.

Note that FIGS. 24A to 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, FIGS.27A to 27D, FIGS. 28A to 28D, FIGS. 29A to 29D, FIGS. 30A to 30D, andFIGS. 31A to 31D illustrate circuits corresponding to the circuits inFIGS. 1A to 1D and the like, to which the switch 914 is added; however,circuits to which the switch 914 is added are not limited to those inFIGS. 1A to 1D and the like. The circuits in the figures other thanFIGS. 1A to 1D may include the switch 914 like the circuits in FIGS. 24Ato 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, FIGS. 27A to 27D, FIGS. 28Ato 28D, FIGS. 29A to 29D, FIGS. 30A to 30D, and FIGS. 31A to 31D. Forexample, the circuits in FIGS. 11A to 11D, to which the switch 14 isadded, may additionally include the switch 914 like the circuits inFIGS. 24A to 24D, FIGS. 25A to 25D, FIGS. 26A to 26D, FIGS. 27A to 27D,FIGS. 28A to 28D, FIGS. 29A to 29D, FIGS. 30A to 30D, and FIGS. 31A to31D. An example of such a case is illustrated in FIGS. 87A to 87D.

The following shows an example of the case where the circuits in FIGS.1A to 1D and FIGS. 11A to 11D employ a driving method different fromthose shown in FIGS. 5A to 5C, FIGS. 17A to 17C, and the like. When sucha driving method is employed, pixels are preferably connected to eachother with a wiring 23 in the row direction rather than the columndirection. FIGS. 34A to 34D illustrate an example of arrangement of thecircuits 100 illustrated in FIGS. 1A to 1D. FIGS. 34A to 34D eachillustrate an example of the case where a plurality of circuits 100connected to different wirings 21 is connected to a common wiring 23. Inother words, the wiring 23 intersects the wirings 21.

An example of operation according to one aspect of the semiconductordevice of the present invention is described using the circuit 100 inFIG. 34C as an example. For this operation, in the first operation inFIG. 5B or FIG. 17B, when the voltage Vi2−Vi1 is supplied to thecapacitor 102 and the gate-source voltage Vgs101 of the transistor 101becomes the voltage Vi2−Vi1, the potential Vi1 is supplied not throughthe wiring 21 or the switch 14 but through the wiring 23. Therefore thedescription for FIGS. 5A to 5C or FIGS. 17A to 17C can apply to oneaspect of the semiconductor device of the present invention.

The operation of the circuit 100 illustrated in FIG. 34C can be mainlydivided into a first operation, a second operation, a third operation, afourth operation, and a fifth operation. Note that one aspect of anembodiment of the present invention is not limited thereto, and anotheroperation can be added or part of the operation can be omitted.

First, the first operation in the period T11 is described. In the periodT11, as illustrated in FIG. 35A, the switch 11 is off, and the switch 12and the switch 13 are on. Further, the potential Vi1 is supplied to thewiring 23. Thus, in the period T11, the potential of the anode of thelight-emitting element 104 a becomes the potential Vi1, and thegate-source voltage Vgs101 of the transistor 101 becomes the voltageVi2−Vi1. In other words, the transistor 101 and the capacitor 102 areinitialized.

Next, the second operation in the period T12 is described. In the periodT12, as illustrated in FIG. 35B, the switch 11 is off, and the switch 12and the switch 13 are on. In addition, the potential VDD is supplied tothe wiring 23. When the potential VDD is supplied to the wiring 23,electric charge accumulated in the capacitor 102 is released through thetransistor 101, and the potential of the source of the transistor 101 israised. Then, when the transistor 101 is turned off, the release of theelectric charge from the capacitor 102 is stopped. The threshold voltageVth of the transistor 101 is eventually held in the capacitor 102. Thus,in the period T12, the threshold voltage Vth is held in the capacitor102, the potential of the anode of the light-emitting element 104 abecomes the potential Vi2−Vth, and the gate-source voltage Vgs101 of thetransistor 101 becomes the threshold voltage Vth. That is, the thresholdvoltage Vth of the transistor 101 can be acquired.

As described above, the first and second operations can be performedwithout the wiring 21; thus, the first and second operations can beperformed for a long period. Therefore, the threshold voltage of thetransistor 101 can be obtained more accurately, resulting in clearimages with less unevenness.

Next, the third operation in the period T13 is described. In the periodT13, as illustrated in FIG. 35C, the switch 11 and the switch 13 are on,and the switch 12 is off. A predetermined potential, e.g., the potentialVDD or Vi1 is supplied to the wiring 23. In addition, the potential Vi3is supplied to the wiring 21. The potential Vi3 may be at the same levelas the potential Vcat, the potential Vi2, or the potential Vi1. Thus, inthe period T13, the threshold voltage Vth is held in the capacitor 102,the potential of the anode of the light-emitting element 104 a becomesthe potential Vi3, the potential of the gate of the transistor 101becomes the potential Vi3+Vth, and the gate-source voltage Vgs101 of thetransistor 101 becomes the voltage Vth.

Next, the fourth operation in the period T14 is described. In the periodT14, as illustrated in FIG. 35D, the switch 11 is on, and the switch 12and the switch 13 are off. In addition, the potential Vsig is suppliedto the wiring 21. Thus, in the period T14, the threshold voltage Vth isheld in the capacitor 102, a voltage Vsig−Vi3−Vα is held in thecapacitor 103, the potential of the anode of the light-emitting element104 a becomes a potential Vi3+Vα, the potential of the gate of thetransistor 101 becomes the potential Vsig+Vth, and the gate-sourcevoltage Vgs101 of the transistor 101 becomes a voltage Vsig+Vth−Vi3−Vα.

The fifth operation in the period T15 is described. In the period T15,as illustrated in FIG. 36 , the switch 11, the switch 12, and the switch13 are off. Thus, in the period T15, the threshold voltage Vth is heldin the capacitor 102, the voltage Vsig−Vi3−Vα is held in the capacitor103, the potential of the anode of the light-emitting element 104 abecomes the potential Vel, the potential of the gate of the transistor101 becomes the potential Vsig+Vth−Vi3−Vα+Vel and the gate-sourcevoltage Vgs101 of the transistor 101 becomes the voltageVsig+Vth−Vi3−Vα. Thus, current based on the potential Vsig can flow tothe light-emitting element 104 a, so that the light-emitting element 104a can emit light at luminance based on the potential Vsig.

Note that the potential Vel occurs when current is fed to thelight-emitting element 104 a through the transistor 101. Specifically,the potential Vel is between the potential VDD and the potential Vcat.

In the fifth operation, the gate-source voltage Vgs101 of the transistor101 can be set to the voltage Vsig+Vth−Vi3−Vα by taking the thresholdvoltage Vth of the transistor 101 into consideration. Consequently,variations in threshold voltage Vth between the transistors 101 can beprevented from adversely affecting the value of a current supplied tothe light-emitting elements 104 a. Alternatively, even if the transistor101 deteriorates and the threshold voltage Vth is changed, the change inthe threshold voltage Vth can be prevented from adversely affecting thevalue of a current supplied to the light-emitting element 104 a.Therefore, high-quality images with less unevenness can be displayed.

Also in a part of the period T15, the potential of the wiring 23 may becontrolled to prevent current from flowing into the light-emittingelement 104 a and the like, thereby providing a non-lighting period. Forexample, when the potential of the wiring 23 is equal to the potentialof the wiring 24, current can be prevented from flowing.

Note that a switch 14 is not provided in the circuits in FIGS. 34A to34D and FIGS. 35A to 35D, but the present invention is not limitedthereto; a switch 14 may be provided in the circuits in FIGS. 34A to 34Dand FIGS. 35A to 35D similarly to the circuits illustrated in FIGS. 11Ato 16D, 17B to 18B, and 20B to 23B.

Note that a switch 914 is not provided in the circuits in FIGS. 34A to34D and FIGS. 35A to 35D, but the present invention is not limitedthereto; a switch 914 may be provided in the circuits in FIGS. 34A to34D and FIGS. 35A to 35D similarly to the circuits illustrated in FIGS.24A to 31D, 46A to 52D, 57A to 57D, 63B to 63D, 64B to 65A, 66A, 67A,67C, 68A, 68C, 73A to 73D, and 87A to 87D.

In FIGS. 34A to 34D and FIGS. 35A to 35D, operation is performed bychanging the potential of the wiring 23; however, operation can also beperformed by controlling the potentials of a plurality of wirings. Anexample of such a case will be described. The description for FIGS. 34Ato 34D, FIGS. 35A to 35D, FIGS. 5A to 5C, and FIGS. 17A to 17C can applyto one aspect of the semiconductor device of the present invention.FIGS. 32A to 32D each illustrate a structure example of a circuit 100.The circuits 100 in FIGS. 32A to 32D correspond to the circuits 100 inFIGS. 1A to 1D or FIGS. 34A to 34D, FIGS. 35A to 35D, and FIG. 36 , towhich a switch 814 and a switch 15 are added and which include a wiring23 a and a wiring 23 b instead of the wiring 23. In FIGS. 32A to 32D,the switch 814 has a function of controlling conduction between theother of the source and the drain of a transistor 101 and the wiring 23a. Further, the switch 15 has a function of controlling conductionbetween the other of the source and the drain of a transistor 101 andthe wiring 23 b.

Note that the wiring 23 a and/or the wiring 23 b can be provided so asto intersect with the wiring 21 or provided in parallel to the wiring21.

A semiconductor device according to one aspect of the present inventionmay include, for example, any circuit that supplies a constant voltageor a signal to the circuit 100, as well as any of the circuits 100illustrated in FIGS. 32A to 32D.

Semiconductor devices illustrated in FIGS. 33A to 33D include, inaddition to the circuits 100 in FIGS. 32A to 32D, a circuit 201 having afunction of supplying a constant voltage or a signal to the wiring 21, acircuit 202 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 203 a having a function of supplyinga constant voltage or a signal to the wiring 23 a, a circuit 203 bhaving a function of supplying a constant voltage or a signal to thewiring 23 b, and a circuit 204 having a function of supplying a constantvoltage or a signal to the wiring 24. Specifically, the circuit 203 ahas a function of supplying the potential Vi1 to the wiring 23 a. Thecircuit 203 b has a function of supplying, for example, a supply voltage(a high supply potential or a low supply potential), e.g., the potentialVDD or the potential VSS to the wiring 23 b. Examples of the circuits203 a and 203 b are power supply circuits.

Accordingly, the wiring 23 a has a function of transmitting or supplyingthe potential Vi1. Alternatively, the wiring 23 a functions as aninitialization line. Note that the potential of the wiring 23 a ispreferably constant; however, one aspect of the embodiment of thepresent invention is not limited thereto. The potential of the wiring 23a may vary like a pulse signal.

Accordingly, the wiring 23 b has a function of transmitting or supplyinga power supply potential. Alternatively, the wiring 23 b has a functionof supplying current to the transistor 101. Alternatively, the wiring 23b has a function of supplying current to the load 104. The wiring 23 bfunctions as a power supply line. Alternatively, the wiring 23 bfunctions as a current supply line. Note that the potential of thewiring 23 b is preferably constant; however, one aspect of theembodiment of the present invention is not limited thereto. Thepotential of the wiring 23 b may vary like a pulse signal. For example,the potential of the wiring 23 b may be a potential at which not onlyforward bias voltage but also reverse bias voltage is applied to theload 104.

Note that in FIGS. 33A to 33D, the semiconductor devices each include,in addition to the circuit 100, the circuits 201, 202, 203 a, 203 b, and204, as an example. However, a semiconductor device according to oneaspect of the present invention does not necessarily include all of thecircuits 201, 202, 203 a, 203 b, and 204, and may include only one orsome of these circuits.

Note that in the circuits 100 in FIGS. 32A to 32D and FIGS. 33A to 33D,as an example, the switch 11, the switch 12, the switch 13, the switch814, and the switch 15 may be transistors.

The circuits 100 in FIGS. 32A to 32D and FIGS. 33A to 33D can operate ina similar way to the circuits 100 in FIGS. 34A to 34D, FIGS. 35A to 35D,and FIG. 36 . Note that in the case of the circuits 100 in FIGS. 32A to32D and FIGS. 33A to 33D, the switch 814 is on and the switch 15 is offin the period T11. Further, the switch 814 is off and the switch 15 ison in the periods T12 to T15.

Note that the circuits 100 in FIG. 32B and FIG. 33B may each furtherinclude a capacitor 105 connected to the load 104 as in FIGS. 8A to 8D,FIGS. 9A to 9D, FIGS. 10A and 10B, FIGS. 21A to 21E, and FIGS. 22A to22E. Similarly, the circuits 100 in FIG. 32C and FIG. 33C may eachfurther include a capacitor 105 connected to the light-emitting element104 a. Similarly, the circuits 100 in FIG. 32D and FIG. 33D may eachfurther include a capacitor 105 connected to the light-emitting element104 b. Specifically, one electrode of the capacitor 105 is connected tothe other electrode of the capacitor 103 and the one of a source and adrain of the transistor 101. The other electrode of the capacitor 105 isconnected to a wiring 26, 24, 23, or 22 which is additionally provided.

Any of the circuits 100 in FIGS. 32B to 32D and the circuitscorresponding to the circuits 100 in FIGS. 33B to 33D to which thecapacitor 105 is added may be used as a pixel of a display device. Inthe case where pixels for a plurality of hues are provided in thedisplay device, the transistors 101 of pixels for different hues maydiffer in the ratio between the channel width and the channel length.

Note that a switch 14 is not provided in the circuits in FIGS. 32A to32D and FIGS. 33A to 33D, but the present invention is not limitedthereto; a switch 14 may be provided in the circuits in FIGS. 34A to 34Dand FIGS. 35A to 35D as those in FIGS. 11A to 16D, 17B to 18B, and 20Bto 23B.

Note that a switch 914 is not provided in the circuits in FIGS. 32A to32D and FIGS. 33A to 33D, but the present invention is not limitedthereto; a switch 914 may be provided in the circuits in FIGS. 32A to32D and FIGS. 33A to 33D as those in FIGS. 24A to 31D, 46A to 52D, 57Ato 57D, 63B to 63D, 64B to 65A, 66A, 67A, 67C, 68A, 68C, 73A to 73D, and87A to 87D.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Embodiment 3

In this embodiment, structure examples of the circuit 100 that is asemiconductor device according to one aspect of the present inventionare described. In this embodiment, a switch or a wiring is added,connection is partly changed, a wiring is connected to a differentwiring to be merged into one wiring, or a driving method is partlychanged in the circuit described in Embodiments 1 and 2, for example.Thus, the contents described in Embodiments 1 and 2 can also be appliedto this embodiment.

The structure of the circuit 100 in FIG. 43A corresponds to a structurewhere the position of the switch 14 is different from that in thecircuit 100 in FIG. 11C or a structure where the switch 14 is added tothe circuit 100 in FIG. 1C. In the circuit 100 in FIG. 43A, the switch14 has a function of controlling conduction between one electrode of thecapacitor 102 or 103 and the wiring 25.

Note that operations are similar to the operations in FIGS. 17A to 17C,FIGS. 18A and 18B, FIGS. 19A to 19D, and FIGS. 20A and 20B. In FIG. 18A,the switch 14 is on in the fourth operation; however, in FIG. 43A, theswitch 14 is preferably off in the fourth operation. Note that oneaspect of the embodiment of the present invention is not limitedthereto.

Note that in FIG. 43A, the switch 914 can be provided as in FIGS. 24A to24D, FIGS. 25A to 25D, FIGS. 26A to 26D, FIGS. 27A to 27D, FIGS. 28A to28D, FIGS. 29A to 29D, FIGS. 30A to 30D, FIGS. 31A to 31D, and the like.In FIG. 43A, the switch 814 and the switch 15 can be provided as inFIGS. 32A to 32D, FIGS. 33A to 33D, and the like. In FIG. 43A, thepotential of the wiring 23 can be controlled as in FIGS. 34A to 34D andthe like. FIG. 43F illustrates an example in which the switch 914 isprovided in FIG. 43A as in FIGS. 30A to 30D.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the capacitor 105 can be added to the circuit 100 inFIG. 43A. For example, the structure of the circuit 100 in FIG. 43Bcorresponds to a structure where the capacitor 105 is added to thecircuit 100 in FIG. 43A. One electrode of the capacitor 105 is connectedto the other electrode of the capacitor 103. The other electrode of thecapacitor 105 is connected to the wiring 26.

Note that as in FIGS. 8A to 8D, FIGS. 21A to 21E, and the like, thewiring 26 can be connected to a variety of wirings. The circuit 100 inFIG. 43C is an example in which the wiring 26 is connected to the wiring25 in the circuit 100 in FIG. 43B. The wiring 26 can be connected to avariety of wirings, for example, the wiring 24, the wiring 22, thewiring 23, the gate signal line, or a wiring of another circuit 100instead of the wiring 25.

Note that the wiring 25 can be connected to a variety of wirings. Thecircuit 100 in FIG. 43D is an example in which the wiring 25 isconnected to the wiring 24 in the circuit 100 in FIG. 43A.

The structure of the circuit 100 in FIG. 43E corresponds to a structurewhere the capacitor 105 is added to the circuit 100 in FIG. 43D. Oneelectrode of the capacitor 105 is connected to the other electrode ofthe capacitor 103. The other electrode of the capacitor 105 is connectedto the wiring 26.

Semiconductor devices in FIGS. 44A to 44F include the circuit 220 havinga function of supplying a constant voltage or a signal to the wiring 21,the circuit 221 having a function of supplying a constant voltage or asignal to the wiring 22, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 43A to 43F.

Note that FIGS. 43A to 43F and FIGS. 44A to 44F each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 43A to 43F and FIGS. 44A to 44F.

Note that as in FIG. 43D, the wiring 25 can be connected to a variety ofwirings as in FIGS. 11A to 11D or the like. The circuit 100 in FIG. 45Ais an example in which the wiring 25 is connected to the wiring 24 inthe circuit 100 in FIG. 11C.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the capacitor 105 can be added or a wiring can beconnected to the circuit 100 in FIG. 45A. For example, the structure ofthe circuit 100 in FIG. 45B corresponds to a structure where thecapacitor 105 is added to the circuit 100 in FIG. 45A. One electrode ofthe capacitor 105 is connected to the other electrode of the capacitor103. The other electrode of the capacitor 105 is connected to the wiring26. Note that the wiring 25 can be connected not to the wiring 24 but tothe wiring 26. Alternatively, the wiring 26 and the wiring 25 can beconnected to the wiring 24.

Semiconductor devices in FIGS. 45C and 45D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 221 having a function of supplyinga constant voltage or a signal to the wiring 22, the circuit 222 havinga function of supplying a constant voltage or a signal to the wiring 23,the circuit 223 having a function of supplying a constant voltage or asignal to the wiring 24, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 45A and 45B.

Note that FIGS. 45A to 45D each illustrate the structure of the circuit100 using the light-emitting element 104 a. However, the semiconductordevice according to one aspect of the present invention may have astructure where the light-emitting element 104 a is not provided or theload 104 or the light-emitting element 104 b is provided instead of thelight-emitting element 104 a in the circuit 100 in each of FIGS. 45A to45D.

Note that in the circuit in FIGS. 1A to 1D or the like, both the switch14 and the switch 914 can be provided by additional provision of theswitch 14 or the switch 914 or both. In other words, the switch 914 canbe added to FIGS. 11A to 11D, FIGS. 32A to 32D, FIGS. 34A to 34D, FIGS.43A to 43F, FIGS. 45A to 45D, or the like or the switch 14 can be addedto FIGS. 24A to 24D, FIGS. 26A to 26D, FIGS. 28A to 28D, FIGS. 30A to30D, FIGS. 32A to 32D, FIGS. 34A to 34D, or the like. For example, thestructure of the circuit 100 in FIG. 46A corresponds to a structurewhere the switch 914 is added to the circuit 100 in FIG. 11C or astructure where the switch 14 is added to the circuit 100 in FIG. 28C.In the circuit 100 in FIG. 46A, the switch 914 has a function ofcontrolling conduction between the one of the source and the drain ofthe transistor 101 and the other electrode of the capacitor 103 or theanode of the light-emitting element 104 a.

Note that as in FIG. 44D, FIGS. 45A to 45D, and the like, the wiring 25can be connected to another wiring in FIG. 46A. FIG. 46B illustrates anexample in which the wiring 25 is connected to the wiring 24 in thecircuit 100 in FIG. 46A.

In addition, the position of the switch 14 is not limited to theposition shown in FIG. 46A, and the switch 14 can be provided in anotherposition as in FIGS. 43A to 43F. The structure of the circuit 100 inFIG. 46C corresponds to a structure where the switch 14 is provided inFIG. 46A as in FIG. 43A. The switch 14 has a function of controllingconduction between one electrode of the capacitor 102 and the wiring 25,and conduction between one electrode of the capacitor 103 and the wiring25.

Further, the number of switches corresponding to the switch 14 is notlimited to one and can be more than one. For example, the structure ofthe circuit 100 in FIG. 46D differs from the structure of the circuit100 in FIG. 46A in that a switch 14 a having a function of controllingconduction between the other electrode of the capacitor 103 and thewiring 24, and conduction between the anode of the light-emittingelement 104 a and the wiring 24 and a switch 14 b having a function ofcontrolling conduction between one electrode of the capacitor 102 andthe wiring 25, and conduction between the one electrode of the capacitor103 and the wiring 25 are provided instead of the switch 14. That is,the number of the switches 14 is two in FIG. 46D.

Semiconductor devices in FIGS. 47A to 47D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 221 having a function of supplyinga constant voltage or a signal to the wiring 22, the circuit 222 havinga function of supplying a constant voltage or a signal to the wiring 23,the circuit 223 having a function of supplying a constant voltage or asignal to the wiring 24, and the circuit 224 having a function ofsupplying a constant voltage or a signal to the wiring 25, in additionto the circuits 100 in FIGS. 46A to 46D.

Note that FIGS. 46A to 46D and FIGS. 47A to 47D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 46A to 46D and FIGS. 47A to 47D.

Note that the switch 14 and the switch 914 are not necessarily providedin the manner illustrated in FIGS. 46A to 46D and FIGS. 87A to 87D. Theswitch 14 and the switch 914 can be provided in a variety of ways. Adriving method in that case can be similar to those in FIGS. 5A to 5C,FIGS. 6A to 6C, FIGS. 17A to 17C, FIGS. 18A and 18B, FIGS. 20A and 20B,FIGS. 35A to 35D, and FIG. 36 . The structure of the circuit 100 in FIG.48A corresponds to a structure where the position of the switch 14 isdifferent from that in the circuit 100 in FIG. 46A. In the circuit 100in FIG. 48A, the switch 14 has a function of controlling conductionbetween the one of the source and the drain of the transistor 101 andthe wiring 25.

Further, in FIG. 48A, the wiring 25 can be connected to another wiringas in FIG. 44D and FIGS. 45A to 45D. The circuit 100 in FIG. 48B is anexample in which the wiring 25 is connected to the wiring 24 in thecircuit 100 in FIG. 46A.

Semiconductor devices in FIGS. 48C and 48D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 221 having a function of supplyinga constant voltage or a signal to the wiring 22, the circuit 222 havinga function of supplying a constant voltage or a signal to the wiring 23,the circuit 223 having a function of supplying a constant voltage or asignal to the wiring 24, and the circuit 224 having a function ofsupplying a constant voltage or a signal to the wiring 25, in additionto the circuits 100 in FIGS. 48A and 48B.

Note that FIGS. 48A to 48D each illustrate the structure of the circuit100 using the light-emitting element 104 a. However, the semiconductordevice according to one aspect of the present invention may have astructure where the light-emitting element 104 a is not provided or theload 104 or the light-emitting element 104 b is provided instead of thelight-emitting element 104 a in the circuit 100 in each of FIGS. 48A to48D.

The structure of the circuit 100 in FIG. 49A is another example in whichthe switch 14 and the switch 914 are provided and corresponds to astructure where the switch 914 is added to the circuit 100 in FIG. 11Cor a structure where the switch 14 is added to the circuit 100 in FIG.30C. In the circuit 100 in FIG. 49A, the switch 914 has a function ofcontrolling conduction between the one of the source and the drain ofthe transistor 101 and the anode of the light-emitting element 104 a,and conduction between the other electrode of the capacitor 103 and theanode of the light-emitting element 104 a.

A driving method in that case can be similar to those in FIGS. 5A to 5C,FIGS. 6A to 6C, FIGS. 17A to 17C, FIGS. 18A and 18B, FIGS. 20A and 20B,FIGS. 35A to 35D, and FIG. 36 . An example of the driving method isdescribed below.

First, first operation in the period T11 is described. In the periodT11, the switch 11 and the switch 914 are off, and the switch 12, theswitch 13, and the switch 14 are on. Thus, in the period T11, thevoltage Vi2−Vi1 is supplied to the capacitor 102, the potential of theanode of the light-emitting element 104 a becomes the potential Vi1, andthe gate-source voltage Vgs101 of the transistor 101 becomes the voltageVi2−Vi1. In other words, the transistor 101 and the capacitor 102 areinitialized.

Note that in the case where the potential of the wiring 21 has noadverse effects on initialization of the transistor 101 and thecapacitor 102, the switch 11 may be on. In that case, the switch 14 maybe off.

Note that the switch 13 may be off.

Note that the switch 914 may be on.

Second operation in the period T12 is described. In the period T12, theswitch 11, the switch 14, and the switch 914 are off, and the switch 12and the switch 13 are on. When the switch 11, the switch 914, and theswitch 14 are turned off, electric charge accumulated in the capacitor102 is released through the transistor 101, and the potential of thesource of the transistor 101 is raised. Then, when the transistor 101 isturned off, the release of the electric charge from the capacitor 102 isstopped. The threshold voltage Vth of the transistor 101 is eventuallyheld in the capacitor 102. Thus, in the period T12, the thresholdvoltage Vth is held in the capacitor 102, the potential of the anode ofthe light-emitting element 104 a becomes the potential Vi2−Vth, and thegate-source voltage Vgs101 of the transistor 101 becomes the thresholdvoltage Vth (or voltage based on Vth). That is, the threshold voltageVth of the transistor 101 (or the voltage based on Vth) can be obtained.

Note that the second operation can be performed regardless of whetherthe value of the threshold voltage Vth of the transistor 101 is positiveor negative. This is because the potential of the source of thetransistor 101 can be raised until the transistor 101 is turned off. Inother words, when the potential of the source of the transistor 101becomes higher than the potential of the gate of the transistor 101, thetransistor 101 can be finally turned off and Vgs101 can become Vth.Thus, the second operation can be performed correctly regardless ofwhether the transistor 101 is an enhancement (normally-off) transistoror a depletion (normally-on) transistor.

Note that when the potential of the anode of the light-emitting element104 a becomes high, it is preferable that current does not flow to thelight-emitting element 104 a. The potential Vi2 is preferably at a lowvalue so that current does not flow to the light-emitting element 104 a.Note that when the switch 914 is turned off, it is possible not tosupply current to the light-emitting element 104 a; thus, the potentialVi2 may be at a high value.

Note that the switch 914 may be on.

Third operation in the period T13 is described. In the period T13, theswitch 11 and the switch 14 are on, and the switch 12, the switch 13,and the switch 914 are off. In addition, the potential Vsig is suppliedto the wiring 21. Thus, in the period T13, the threshold voltage Vth (orthe voltage based on Vth) is held in the capacitor 102, the voltageVsig−Vi1 is held in the capacitor 103, the potential of the anode of thelight-emitting element 104 a becomes the potential Vi1, the potential ofthe gate of the transistor 101 becomes the potential Vsig+Vth, and thegate-source voltage Vgs101 of the transistor 101 becomes the voltageVsig+Vth−Vi1. Thus, the potential Vsig can be input to the capacitor103. Alternatively, the sum of voltage across the capacitor 102 andvoltage across the capacitor 103 can be equal to the gate-source voltageof the transistor 101.

Note that in that case, the switch 14 can be turned off.

Note that the switch 914 may be on.

Fourth operation in the period T14 is described. In the period T14, theswitch 11, the switch 12, the switch 13, and the switch 14 are off, andthe switch 914 is on. Thus, in the period T14, the threshold voltage Vthis held in the capacitor 102, the voltage Vsig−Vi1 is held in thecapacitor 103, the potential of the anode of the light-emitting element104 a becomes the potential Vel, the potential of the gate of thetransistor 101 becomes the potential Vsig+Vth+Vel, and the gate-sourcevoltage Vgs101 of the transistor 101 becomes the voltage Vsig+Vth−Vi1.Thus, current based on the potential Vsig can flow to the light-emittingelement 104 a, so that the light-emitting element 104 a can emit lightat luminance based on the potential Vsig.

Note that in part of the period in the fourth operation, it is possibleto forcibly turn off the transistor 101 or not to supply current to thelight-emitting element 104 a so that the light-emitting element 104 adoes not emit light. In other words, a non-lighting period can beprovided. For example, by turning on the switch 12, the transistor 101can be turned off. Alternatively, by turning on the switch 14, it ispossible not to supply current to the light-emitting element 104 a.Alternatively, by turning off the switch 914, it is possible not tosupply current to the light-emitting element 104 a.

Note that the period T16 during which the sixth operation is performedmay be provided after the period T13 during which the third operation isperformed and before the period T14 during which the fourth operation isperformed.

The sixth operation in the period T16 is described. In the period T16,the switch 12 is on, and the switch 11, the switch 13, the switch 914,and the switch 14 are off. Thus, in the period T16, the gate-sourcevoltage Vgs101 of the transistor 101 becomes the voltageVsig+Vth−Vi1−Vα.

The potential Vα in the sixth operation varies when the anode of thelight-emitting element 104 a enters into an electrically floating state.The value of the potential Vα depends on the ratio between thecapacitance of the light-emitting element 104 a and the capacitance ofthe capacitor 102 and the capacitor 103 if the transistor 101 is off.However, depending on the level of the potential Vsig, the transistor101 might be turned on and electric charge flows into the anode of thelight-emitting element 104 a through the transistor 101. Thus, the valueof the potential Vα depends not only on the capacitance ratio but alsoon the electric charge flowing into the anode of the light-emittingelement 104 a.

Note that the on/off state of the switch 12 and the on/off state of theswitch 13 can be controlled at the same timing. Thus, in the case wherethe switch 12 and the switch 13 are transistors having the samepolarity, gates of the transistors can be connected to each other as inFIGS. 15A to 15D.

The wiring 22 or 25 can be connected to a variety of wirings. Thecircuit 100 in FIG. 49B is an example in which the wiring 25 isconnected to the wiring 24 in the circuit 100 in FIG. 49A.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the capacitor 105 can be added. For example, thestructure of the circuit 100 in FIG. 49C corresponds to a structurewhere the capacitor 105 is added to the circuit 100 in FIG. 49A. Oneelectrode of the capacitor 105 is connected to the other electrode ofthe capacitor 103. The other electrode of the capacitor 105 is connectedto the wiring 26.

As an example in which the placement of the capacitor 105 is differentfrom that in FIG. 49C, the structure of the circuit 100 in FIG. 49Dcorresponds to a structure where the capacitor 105 is added to thecircuit 100 in FIG. 49A. One electrode of the capacitor 105 is connectedto the anode of the light-emitting element 104 a. The other electrode ofthe capacitor 105 is connected to the wiring 26.

Semiconductor devices in FIGS. 50A to 50D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 221 having a function of supplyinga constant voltage or a signal to the wiring 22, the circuit 222 havinga function of supplying a constant voltage or a signal to the wiring 23,the circuit 223 having a function of supplying a constant voltage or asignal to the wiring 24, the circuit 224 having a function of supplyinga constant voltage or a signal to the wiring 25, and the circuit 225having a function of supplying a constant voltage or a signal to thewiring 26, in addition to the circuits 100 in FIGS. 49A to 49D.

Note that FIGS. 49A to 49D and FIGS. 50A to 50D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 49A to 49D and FIGS. 50A to 50D.

Note that as in FIGS. 8A to 8D, FIGS. 21A to 21E, FIGS. 43A to 43F,FIGS. 45A to 45D, FIGS. 46A to 46D, and the like, the wiring 22, thewiring 23, the wiring 24, the wiring 25, the wiring 26, and the like canbe connected to each other in FIGS. 49A to 49D.

Note that one capacitor 105 is added to each of FIGS. 49C and 49D;however, one aspect of the embodiment of the present invention is notlimited thereto. More capacitors can be added to the circuit 100 towhich the capacitor 105 is added. For example, the structure of thecircuit 100 in FIG. 51A corresponds to a structure where a capacitor 105a and a capacitor 105 b are added to the circuit 100 in FIG. 49A. Oneelectrode of the capacitor 105 a is connected to the other electrode ofthe capacitor 103. The other electrode of the capacitor 105 a isconnected to the wiring 26. One electrode of the capacitor 105 b isconnected to the anode of the light-emitting element 104 a. The otherelectrode of the capacitor 105 b is connected to a wiring 27.

The wiring 25 can be connected to another wiring. For example, thestructure of the circuit 100 in FIG. 51B corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 50C.

The structure of the circuit 100 in FIG. 51C corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 50D.

The structure of the circuit 100 in FIG. 51D corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 51A.

Semiconductor devices in FIGS. 52A to 52D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 221 having a function of supplyinga constant voltage or a signal to the wiring 22, the circuit 222 havinga function of supplying a constant voltage or a signal to the wiring 23,the circuit 223 having a function of supplying a constant voltage or asignal to the wiring 24, the circuit 224 having a function of supplyinga constant voltage or a signal to the wiring 25, the circuit 225 havinga function of supplying a constant voltage or a signal to the wiring 26,and a circuit 226 having a function of supplying a constant voltage or asignal to the wiring 27, in addition to the circuits 100 in FIGS. 51A to51D.

An example of the circuit 226 is a power supply circuit. Accordingly,the wiring 27 has a function of capable of transmitting or supplying apredetermined potential. Alternatively, the wiring 27 functions as acapacitance line. Note that the potential of the wiring 27 is preferablyconstant; however, one aspect of the embodiment of the present inventionis not limited thereto. The potential of the wiring 27 may vary like apulse signal. The wiring 27 can be connected to another wiring. Forexample, the wiring 27 can be connected to a variety of wirings, forexample, the wiring 25, the wiring 24, the wiring 22, the wiring 26, thewiring 23, the gate signal line, or a wiring of another circuit 100.

Note that FIGS. 51A to 51D and FIGS. 52A to 52D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 51A to 51D and FIGS. 52A to 52D.

Note that in the variety of circuits, the wiring 22 can be connected toanother wiring. Accordingly, the number of wirings can be reduced. Forexample, the wiring 22 can be connected to the wiring 21, the wiring 23,the wiring 23 a, the wiring 23 b, the wiring 24, the wiring 25, thewiring 26, the wiring 27, or the like. Alternatively, the wiring 22 canbe connected to the scan line, the gate line, a wiring connected to thegate of the transistor, or the like. For example, the structure of thecircuit 100 in FIG. 53A corresponds to a structure where the wiring 22is connected to the wiring 21 in the circuit 100 in FIG. 11C.

Similarly, the structure of the circuit 100 in FIG. 53B corresponds to astructure where the wiring 22 is connected to the wiring 21 in thecircuit 100 in FIG. 1C.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the capacitor 105 can be added. For example, thestructure of the circuit 100 in FIG. 53C corresponds to a structurewhere the capacitor 105 is added to the circuit 100 in FIG. 53A or astructure where the wiring 22 is connected to the wiring 21 in thecircuit 100 in FIG. 21A.

As in FIGS. 43A to 43F and the like, the wiring 22 can be connected tothe wiring 21. For example, the structure of the circuit 100 in FIG. 53Dcorresponds to a structure where the wiring 22 is connected to thewiring 21 in the circuit 100 in FIG. 43B.

Semiconductor devices in FIGS. 54A to 54D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 53A to 53D.

Note that FIGS. 53A to 53D and FIGS. 54A to 54D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 53A to 53D and FIGS. 54A to 54D.

Note that in the case where one wiring is connected to a first wiring, asecond wiring can be connected to the first wiring or a third wiring.For example, in the case where the wiring 22 is connected to one wiring,the wiring 25 can be connected to another wiring. For example, thestructure of the circuit 100 in FIG. 55A corresponds to a structurewhere the wiring 22 is connected to the wiring 21 and the wiring 25 isconnected to the wiring 24 in the circuit 100 in FIG. 11C, a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 54A, or a structure where the wiring 22 is connected to the wiring21 in the circuit 100 in FIG. 45A.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the capacitor 105 can be added. For example, thestructure of the circuit 100 in FIG. 55B corresponds to a structurewhere the capacitor 105 is added to the circuit 100 in FIG. 55A or astructure where the wiring 22 is connected to the wiring 21 in thecircuit 100 in FIG. 45B.

Further, the position of the switch 14 can be changed. The structure ofthe circuit 100 in FIG. 55C corresponds to a structure where the wiring22 is connected to the wiring 21 in the circuit 100 in FIG. 43E.

Semiconductor devices in FIGS. 56A to 56C include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,and the circuit 225 having a function of supplying a constant voltage ora signal to the wiring 26, in addition to the circuits 100 in FIGS. 55Ato 55C.

Note that FIGS. 55A to 55C and FIGS. 56A to 56C each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 55A to 55C and FIGS. 56A to 56C.

The structure of the circuit 100 in FIG. 57A corresponds to a structurewhere the wiring 22 is connected to the wiring 21 in the circuit 100 inFIG. 46A.

The structure of the circuit 100 in FIG. 57B corresponds to a structurewhere the wiring 22 is connected to the wiring 21 in the circuit 100 inFIG. 28C.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the switch 914 can be added. For example, thestructure of the circuit 100 in FIG. 57C corresponds to a structurewhere the 914 is added to the circuit 100 in FIG. 53C.

The structure of the circuit 100 in FIG. 57D corresponds to a structurewhere the switch 914 is added to the circuit 100 in FIG. 53D or astructure where the placement of the switch 14 in FIG. 57C is changed.

Semiconductor devices in FIGS. 58A to 58D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 57A to 57D.

Note that FIGS. 57A to 57D and FIGS. 58A to 58D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 57A to 57D and FIGS. 58A to 58D.

The structure of the circuit 100 in FIG. 59A corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 57A.

The structure of the circuit 100 in FIG. 59B corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 57C.

The structure of the circuit 100 in FIG. 59C corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 57D.

Semiconductor devices in FIGS. 60A to 60C include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,and the circuit 225 having a function of supplying a constant voltage ora signal to the wiring 26, in addition to the circuits 100 in FIGS. 59Ato 59C.

Note that FIGS. 59A to 59C and FIGS. 60A to 60C each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 59A to 59C and FIGS. 60A to 60C.

Note that the wiring 22 can be connected to a wiring other than thewiring 21. For example, the wiring 22 can be connected to the wiring 24.For example, the structure of the circuit 100 in FIG. 61A corresponds toa structure where the wiring 22 is connected to the wiring 24 in thecircuit 100 in FIG. 11C.

The structure of the circuit 100 in FIG. 61B corresponds to a structurewhere the wiring 22 is connected to the wiring 24 in the circuit 100 inFIG. 1C.

The structure of the circuit 100 in FIG. 61C corresponds to a structurewhere the wiring 22 is connected to the wiring 24 in the circuit 100 inFIG. 21A.

The structure of the circuit 100 in FIG. 61D corresponds to a structurewhere the wiring 22 is connected to the wiring 24 in the circuit 100 inFIG. 43A.

Semiconductor devices in FIGS. 62A to 62D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 61A to 61D.

Note that FIGS. 61A to 61D and FIGS. 62A to 62D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 61A to 61D and FIGS. 62A to 62D.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the capacitor 105 can be added. For example, thestructure of the circuit 100 in FIG. 63A corresponds to a structurewhere the capacitor 105 is added to the circuit 100 in FIG. 61B or astructure where the wiring 22 is connected to the wiring 24 in thecircuit 100 in FIG. 8A.

The structure of the circuit 100 in FIG. 63B corresponds to a structurewhere the wiring 22 is connected to the wiring 24 in the circuit 100 inFIG. 46A.

The structure of the circuit 100 in FIG. 63C corresponds to a structurewhere the wiring 22 is connected to the wiring 24 in the circuit 100 inFIG. 28C.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the structure of the circuit 100 in FIG. 63Dcorresponds to a structure where the capacitor 105 is added to thecircuit 100 in FIG. 63C.

Semiconductor devices in FIGS. 64A to 64D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 63A to 63D.

Note that FIGS. 63A to 63D and FIGS. 64A to 64D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 63A to 63D and FIGS. 64A to 64D.

The structure of the circuit 100 in FIG. 65A corresponds to a structurewhere the switch 914 is added to the circuit 100 in FIG. 61D. In thecircuit 100 in FIG. 65A, the switch 914 has a function of controllingconduction between the one of the source and the drain of the transistor101 and the other electrode of the capacitor 103, and conduction betweenthe one of the source and the drain of the transistor 101 and the anodeof the light-emitting element 104 a.

The structure of the circuit 100 in FIG. 65B corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 61A.

The structure of the circuit 100 in FIG. 65C corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 61C.

The structure of the circuit 100 in FIG. 65D corresponds to a structurewhere the wiring 25 is connected to the wiring 24 in the circuit 100 inFIG. 61D.

Semiconductor devices in FIGS. 66A to 66D include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 65A to 65D.

Note that FIGS. 65A to 65D and FIGS. 66A to 66D each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 65A to 65D and FIGS. 66A to 66D.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the structure of the circuit 100 in FIG. 67Acorresponds to a structure where the capacitor 105 is added to thecircuit 100 in FIG. 65A.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the structure of the circuit 100 in FIG. 67Bcorresponds to a structure where the capacitor 105 is added to thecircuit 100 in FIG. 65D.

As in FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 21A to 21E, FIGS. 22A to22E, and the like, the structure of the circuit 100 in FIG. 67Ccorresponds to a structure where the capacitor 105 is added to thecircuit 100 in FIG. 63C.

Semiconductor devices in FIGS. 68A to 68C include more than one of thecircuit 220 having a function of supplying a constant voltage or asignal to the wiring 21, the circuit 222 having a function of supplyinga constant voltage or a signal to the wiring 23, the circuit 223 havinga function of supplying a constant voltage or a signal to the wiring 24,the circuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, and the circuit 225 having a function ofsupplying a constant voltage or a signal to the wiring 26, in additionto the circuits 100 in FIGS. 67A to 67C.

Note that FIGS. 67A to 67C and FIGS. 68A to 68C each illustrate thestructure of the circuit 100 using the light-emitting element 104 a.However, the semiconductor device according to one aspect of the presentinvention may have a structure where the light-emitting element 104 a isnot provided or the load 104 or the light-emitting element 104 b isprovided instead of the light-emitting element 104 a in the circuit 100in each of FIGS. 67A to 67C and FIGS. 68A to 68C.

Note that as in the diagrams that have been described, the switch 14,the switch 914, the capacitor 105, or the like can be added to thecircuit 100 in each of FIGS. 32A to 32D. Alternatively, in the circuits100 in FIGS. 32A to 32D, the variety of wirings are connected to avariety of different wirings so that the number of wirings can bereduced. For example, as in FIGS. 11A to 11D, the structures of thecircuits 100 in FIGS. 69A to 69D correspond to structures where theswitches 14 are added to the circuits 100 in FIGS. 32A to 32D.

The semiconductor device according to one aspect of the presentinvention may further include a circuit for supplying a variety ofconstant voltage or signals to the circuit 100, in addition to thecircuits 100 in FIGS. 69A to 69D.

Semiconductor devices in FIGS. 70A to 70D include the circuit 220 havinga function of supplying a constant voltage or a signal to the wiring 21,the circuit 221 having a function of supplying a constant voltage or asignal to the wiring 22, a circuit 222 a having a function of supplyinga constant voltage or a signal to the wiring 23 a, a circuit 222 bhaving a function of supplying a constant voltage or a signal to thewiring 23 b, the circuit 223 having a function of supplying a constantvoltage or a signal to the wiring 24, and the circuit 224 having afunction of supplying a constant voltage or a signal to the wiring 25,in addition to the circuits 100 in FIGS. 69A to 69D.

Note that FIGS. 70A to 70D each illustrate an example in which thesemiconductor device includes the circuit 220, the circuit 221, thecircuit 222 a, the circuit 222 b, the circuit 223, and the circuit 224in addition to the circuit 100. However, the semiconductor deviceaccording to one aspect of the present invention does not necessarilyinclude all the circuit 220, the circuit 221, the circuit 222 a, thecircuit 222 b, the circuit 223, and the circuit 224 in addition to thecircuit 100. The semiconductor device may include only one or more ofthese circuits.

The structure of the circuit 100 in FIG. 73A corresponds to a structurewhere the switch 914 is added to the circuit 100 in FIG. 32C. The switch914 has a function of controlling conduction between the one of thesource and the drain of the transistor 101 and the other electrode ofthe capacitor 103, and conduction between the one of the source and thedrain of the transistor 101 and the anode of the light-emitting element104 a.

The structure of the circuit 100 in FIG. 73B corresponds to a structurewhere the switch 914 is added to the circuit 100 in FIG. 69C. The switch914 has a function of controlling conduction between the one of thesource and the drain of the transistor 101 and the other electrode ofthe capacitor 103, and conduction between the one of the source and thedrain of the transistor 101 and the anode of the light-emitting element104 a.

The semiconductor device according to one aspect of the presentinvention may further include a circuit for supplying a variety ofconstant voltage or signals to the circuit 100, in addition to thecircuits 100 in FIGS. 73A and 73B.

Semiconductor devices in FIGS. 73C and 73D include the circuit 220having a function of supplying a constant voltage or a signal to thewiring 21, the circuit 221 having a function of supplying a constantvoltage or a signal to the wiring 22, the circuit 222 a having afunction of supplying a constant voltage or a signal to the wiring 23 a,the circuit 222 b having a function of supplying a constant voltage or asignal to the wiring 23 b, the circuit 223 having a function ofsupplying a constant voltage or a signal to the wiring 24, and thecircuit 224 having a function of supplying a constant voltage or asignal to the wiring 25, in addition to the circuits 100 in FIGS. 73Aand 73B.

Note that FIGS. 73C and 73D each illustrate an example in which thesemiconductor device includes the circuit 220, the circuit 221, thecircuit 222 a, the circuit 222 b, the circuit 223, and the circuit 224in addition to the circuit 100. However, the semiconductor deviceaccording to one aspect of the present invention does not necessarilyinclude all the circuit 220, the circuit 221, the circuit 222 a, thecircuit 222 b, the circuit 223, and the circuit 224 in addition to thecircuit 100. The semiconductor device may include only one or more ofthese circuits.

Note that as in the diagrams that have been described, the switch 14,the switch 914, the capacitor 105, or the like can be added to thecircuit 100 in each of FIGS. 34A to 34D. Alternatively, in the circuits100 in FIGS. 34A to 34D, the variety of wirings are connected to avariety of different wirings so that the number of wirings can bereduced. For example, FIGS. 71A to 71D illustrate examples of theplacement of the circuits 100 in FIGS. 34A to 34D.

The operation of the semiconductor device according to one aspect of thepresent invention is described taking the circuit 100 in FIG. 71C as anexample.

The operation of the circuit 100 in FIG. 71C can be mainly divided intofirst operation, second operation, third operation, and fourthoperation. Note that the operation of the circuit 100 in FIG. 71C is notlimited thereto, and another operation can be added or part of theoperation can be omitted.

First, the first operation in the period T11 is described. In the periodT11, as illustrated in FIG. 72A, the switch 11, the switch 13, and theswitch 14 are off, and the switch 12 is on. The potential Vi1 issupplied to the wiring 23. Thus, in the period T11, the potential of theanode of the light-emitting element 104 a becomes the potential Vi1, andthe gate-source voltage Vgs101 of the transistor 101 becomes the voltageVi2−Vi1.

Note that although FIG. 72A illustrates the example in which the switch11 is off, the switch 11 may be on. In addition, although FIG. 72Aillustrates the example in which the switch 14 is off, the switch 14 maybe on. In that case, the potential Vi3 is supplied to the wiring 25.Thus, the potential of the anode of the light-emitting element 104 abecomes the potential Vi3, and the gate-source voltage Vgs101 of thetransistor 101 becomes voltage Vi2−Vi3. Further, the switch 13 may beon.

The second operation in the period T12 is described. In the period T12,as illustrated in FIG. 72B, the switch 11 and the switch 14 are off, andthe switch 12 and the switch 13 are on. The potential VDD or a potentialthat is higher than the potential Vi1 is supplied to the wiring 23. Bysupply of the potential VDD to the wiring 23, electric chargeaccumulated in the capacitor 102 is released, and the threshold voltageVth of the transistor 101 is eventually held in the capacitor 102. Thus,in the period T12, the threshold voltage Vth is held in the capacitor102, the anode of the light-emitting element 104 a becomes the potentialVi2−Vth, and the gate-source voltage Vgs101 of the transistor 101becomes the threshold voltage Vth.

The third operation in the period T13 is described. In the period T13,as illustrated in FIG. 72C, the switch 11 and the switch 14 are on, andthe switch 12 and the switch 13 are off. The potential Vsig is suppliedto the wiring 21, the potential VDD is supplied to the wiring 23, andthe potential Vi3 is supplied to the wiring 25. Thus, in the period T13,the threshold voltage Vth is held in the capacitor 102, voltage Vsig−Vi3is held in the capacitor 103, the potential of the anode of thelight-emitting element 104 a becomes the potential Vi3, the potential ofthe gate of the transistor 101 becomes the potential Vsig+Vth, and thegate-source voltage Vgs101 of the transistor 101 becomes voltageVsig+Vth−Vi3. Further, the switch 14 may be off.

The fourth operation in the period T14 is described. In the period T14,as illustrated in FIG. 72D, the switch 11, the switch 12, the switch 13,and the switch 14 are off. The potential VDD is supplied to the wiring23. Thus, in the period T14, the threshold voltage Vth is held in thecapacitor 102, the voltage Vsig−Vi3 is held in the capacitor 103, thepotential of the anode of the light-emitting element 104 a becomes thepotential Vel, the potential of the gate of the transistor 101 becomes apotential Vsig+Vth−Vi3+Vel, and the gate-source voltage Vgs101 of thetransistor 101 becomes the voltage Vsig+Vth−Vi3.

Note that the potential Vel is set when current flows to thelight-emitting element 104 a through the transistor 101. Specifically,the potential Vel is set to a potential between the potential VDD andthe potential Vcat.

In the fourth operation, the gate-source voltage Vgs101 of thetransistor 101 can be set to the voltage Vsig+Vth−Vi3 by taking thethreshold voltage Vth of the transistor 101 into consideration.Accordingly, variations in the threshold voltage Vth of the transistors101 can be prevented from influencing the value of current supplied tothe light-emitting elements 104 a. Alternatively, even when thetransistor 11 is degraded and the threshold voltage Vth is changed, thechange in the threshold voltage Vth can be prevented from influencingthe value of current supplied to the light-emitting element 104 a. Thus,display unevenness can be reduced and high-quality images can bedisplayed.

Note that in the semiconductor device according to one aspect of thepresent invention, the gate of the transistor 101 is held at thepotential Vi2 in the second operation. Accordingly, even when thetransistor 101 is normally on, that is, even when the threshold voltageVth is negative voltage, the electric charge accumulated in thecapacitor 102 can be released until the potential of the source of thetransistor 101 becomes higher than the potential Vi2 of the gate of thetransistor 101. Thus, in the semiconductor device according to oneaspect of the present invention, even when the transistor 101 isnormally on, it is possible to set the gate-source voltage Vgs101 of thetransistor 101 by taking the threshold voltage Vth of the transistor 101into consideration in the fourth operation.

In this embodiment, one wiring is connected to a variety of differentwirings, for example, the wiring 21, the wiring 22, the wiring 23, thewiring 24, the wiring 25, the wiring 26, the wiring 27, a wiring ofanother circuit 100, the scan line, the gate line, a wiring connected tothe gate of the transistor, or the like. Thus, the number of wirings canbe reduced. Further, another switch or another element, for example, theswitch 914, the switch 814, the switch 14, or the capacitor 105 is addedto the circuit 100. In other words, this embodiment is obtained byperforming change, addition, modification, removal, application,superordinate conceptualization, or subordinate conceptualization onpart or all of another embodiment. Thus, part or all of this embodimentcan be freely combined with, applied to, or replaced with part or all ofanother embodiment.

Embodiment 4

FIGS. 74A to 74F, FIGS. 75A to 75E, and FIGS. 76A to 76G each illustrateexamples of the placement of a variety of wirings in a semiconductordevice according to one aspect of the present invention.

In FIG. 74A, a circuit 100(i, j) in an i-th column and a j-th row and acircuit 100(i, j+1) in the i-th column and a (j+1)th row share onewiring 21 and one wiring 23. In addition, a circuit 100(i+1, j) in an(i+1)th column and the j-th row and a circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring23.

In FIG. 74B, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row and the circuit 100(i+1,j) in the (i+1)th column and the j-th row share one wiring 23.Furthermore, the circuit 100(i, j+1) in the i-th column and the (j+1)throw and the circuit 100(i+1, j+1) in the (i+1)th column and the (j+1)throw share one wiring 23.

In FIG. 74C, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row and the circuit 100(i+1,j) in the (i+1)th column and the j-th row share one wiring 23.Furthermore, the circuit 100(i, j+1) in the i-th column and the (j+1)throw and the circuit 100(i+1, j+1) in the (i+1)th column and the (j+1)throw share one wiring 23. Further, the circuit 100(i, j) in the i-thcolumn and the j-th row and the circuit 100(i, j+1) in the i-th columnand the (j+1)th row share one wiring 23. Furthermore, the circuit100(i+1, j) in the (i+1)th column and the j-th row and the circuit100(i+1, j+1) in the (i+1)th column and the (j+1)th row share one wiring23. These wirings 23 are connected to each other.

In FIG. 74D, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 23. The wiring 23 isaligned with the wirings 21.

In FIG. 74E, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 23. The wiring 23intersects the wirings 21.

In FIG. 74F, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share two wirings 23. The two wirings23 intersect and are connected to each other.

In FIG. 75A, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21, one wiring 22, and one wiring 23. In addition, thecircuit 100(i+1, j) in the (i+1)th column and the j-th row and thecircuit 100(i+1, j+1) in the (i+1)th column and the (j+1)th row shareone wiring 21, one wiring 22, and one wiring 23.

In FIG. 75B, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21 and one wiring 22. In addition, the circuit 100(i+1, j) inthe (i+1)th column and the j-th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring22. Further, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i+1, j) in the (i+1)th column and the j-th row shareone wiring 23. Furthermore, the circuit 100(i, j+1) in the i-th columnand the (j+1)th row and the circuit 100(i+1, j+1) in the (i+1)th columnand the (j+1)th row share one wiring 23.

In FIG. 75C, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21 and one wiring 23. In addition, the circuit 100(i+1, j) inthe (i+1)th column and the j-th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring23. Further, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i+1, j) in the (i+1)th column and the j-th row shareone wiring 22. Furthermore, the circuit 100(i, j+1) in the i-th columnand the (j+1)th row and the circuit 100(i+1, j+1) in the (i+1)th columnand the (j+1)th row share one wiring 22.

In FIG. 75D, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row and the circuit 100(i+1,j) in the (i+1)th column and the j-th row share one wiring 22 and onewiring 23. Furthermore, the circuit 100(i, j+1) in the i-th column andthe (j+1)th row and the circuit 100(i+1, j+1) in the (i+1)th column andthe (j+1)th row share one wiring 22 and one wiring 23.

In FIG. 75E, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21 and one wiring 23. In addition, the circuit 100(i+1, j) inthe (i+1)th column and the j-th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring23. Further, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i+1, j) in the (i+1)th column and the j-th row shareone wiring 22 and one wiring 23. Furthermore, the circuit 100(i, j+1) inthe i-th column and the (j+1)th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 22 and one wiring23. These wirings 23 are connected to each other.

In FIG. 76A, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21 and one wiring 22. In addition, the circuit 100(i+1, j) inthe (i+1)th column and the j-th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring22. Further, the circuit 100(i, j) in the i-th column and the j-th row,the circuit 100(i, j+1) in the i-th column and the (j+1)th row, thecircuit 100(i+1, j) in the (i+1)th column and the j-th row, and thecircuit 100(i+1, j+1) in the (i+1)th column and the (j+1)th row shareone wiring 23. The wiring 23 is aligned with the wirings 21 and thewirings 22.

In FIG. 76B, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21 and one wiring 23. In addition, the circuit 100(i+1, j) inthe (i+1)th column and the j-th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring23. Further, the circuit 100(i, j) in the i-th column and the j-th row,the circuit 100(i, j+1) in the i-th column and the (j+1)th row, thecircuit 100(i+1, j) in the (i+1)th column and the j-th row, and thecircuit 100(i+1, j+1) in the (i+1)th column and the (j+1)th row shareone wiring 22. The wiring 22 is aligned with the wirings 21 and thewirings 23.

In FIG. 76C, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 22 and one wiring23. The wiring 22 and the wiring 23 are aligned with the wirings 21.

In FIG. 76D, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 22 and one wiring23. The wiring 22 and the wiring 23 intersect the wirings 21.

In FIG. 76E, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 22 and one wiring23. The wiring 22 intersects the wirings 21. The wiring 23 is alignedwith the wirings 21.

In FIG. 76F, the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21. In addition, the circuit 100(i+1, j) in the (i+1)thcolumn and the j-th row and the circuit 100(i+1, j+1) in the (i+1)thcolumn and the (j+1)th row share one wiring 21. Further, the circuit100(i, j) in the i-th column and the j-th row, the circuit 100(i, j+1)in the i-th column and the (j+1)th row, the circuit 100(i+1, j) in the(i+1)th column and the j-th row, and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 22 and one wiring23. The wiring 23 intersects the wirings 21. The wiring 22 is alignedwith the wirings 21.

In FIG. 76G the circuit 100(i, j) in the i-th column and the j-th rowand the circuit 100(i, j+1) in the i-th column and the (j+1)th row shareone wiring 21 and one wiring 22. In addition, the circuit 100(i+1, j) inthe (i+1)th column and the j-th row and the circuit 100(i+1, j+1) in the(i+1)th column and the (j+1)th row share one wiring 21 and one wiring22. Further, the circuit 100(i, j) in the i-th column and the j-th row,the circuit 100(i, j+1) in the i-th column and the (j+1)th row, thecircuit 100(i+1, j) in the (i+1)th column and the j-th row, and thecircuit 100(i+1, j+1) in the (i+1)th column and the (j+1)th row sharetwo wirings 23. The two wirings 23 intersect and are connected to eachother.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Embodiment 5

An example of a top view of the circuit 100 in FIG. 13A is illustratedin FIG. 77 .

In FIG. 77 , a semiconductor film 300 functions as an active layer ofthe transistor 11 t, one electrode of the capacitor 102, one electrodeof the capacitor 103, an active layer of the transistor 13 t, an activelayer of the transistor 14 t, and an active layer of the transistor 101.A semiconductor film 301 functions as an active layer of the transistor12 t. A conductive film 302 functions as the other electrode of thecapacitor 102. A conductive film 303 functions as the other electrode ofthe capacitor 103. A conductive film 304 functions as the gate of thetransistor 13 t.

A conductive film 305 is connected to the wiring 22 and one of thesource and the drain of the transistor 12 t. A conductive film 306 isconnected to the other of the source and the drain of the transistor 12t and the conductive film 302. A conductive film 307 is connected to theconductive film 304 and the wiring 33. A conductive film 308 isconnected to the conductive film 303, one of the source and the drain ofthe transistor 101, one of a source and a drain of the transistor 13 t,and one of a source and a drain of the transistor 14 t. A conductivefilm 309 is connected to the other of the source and the drain of thetransistor 14 t and the wiring 25.

Note that in FIG. 13B, the load 104 may be connected to the conductivefilm 308. In FIG. 13C, the anode of the light-emitting element 104 a maybe connected to the conductive film 308. In FIG. 13D, the cathode of thelight-emitting element 104 b may be connected to the conductive film308.

Next, another example of the top view of the circuit 100 in FIG. 13A isillustrated in FIG. 78 .

In FIG. 78 , a semiconductor film 320 functions as the active layer ofthe transistor 11 t. A semiconductor film 321 functions as the activelayer of the transistor 12 t. A semiconductor film 322 functions as theactive layer of the transistor 13 t. A semiconductor film 323 functionsas the active layer of the transistor 14 t. A semiconductor film 333functions as the active layer of the transistor 101.

A conductive film 324 functions as the other electrode of the capacitor102 and the gate of the transistor 101. A conductive film 325 functionsas the other electrode of the capacitor 103. A conductive film 326functions as the gate of the transistor 13 t.

A conductive film 327 functions as the one electrode of the capacitor102 and the one electrode of the capacitor 103, and is connected to oneof a source and a drain of the transistor 11 t. A conductive film 328 isconnected to the wiring 22 and one of a source and a drain of thetransistor 12 t. A conductive film 329 is connected to the other of thesource and the drain of the transistor 12 t and the conductive film 324.A conductive film 330 is connected to the conductive film 326 and thewiring 33. A conductive film 331 is connected to the conductive film325, the one of the source and the drain of the transistor 101, one ofthe source and the drain of the transistor 13 t, and one of the sourceand the drain of the transistor 14 t. A conductive film 332 is connectedto the other of the source and the drain of the transistor 14 t and thewiring 25.

Note that in FIG. 13B, the load 104 may be connected to the conductivefilm 331. In FIG. 13C, the anode of the light-emitting element 104 a maybe connected to the conductive film 331. In FIG. 13D, the cathode of thelight-emitting element 104 b may be connected to the conductive film331.

FIG. 80A is an example of a cross-sectional view taken along broken lineA1-A2 in FIG. 78 . FIG. 80B is an example of a cross-sectional viewtaken along broken line B1-B2 in FIG. 78 . In FIGS. 80A and 80B, aninsulating film 801 is formed over a substrate 800, and the wiring 31,the conductive film 324, and the conductive film 325 are formed over theinsulating film 801. An insulating film 802 is formed over the wiring31, the conductive film 324, and the conductive film 325.

The conductive film 327 is formed over the insulating film 802 tooverlap with the conductive film 325. A portion where the conductivefilm 325, the insulating film 802, and the conductive film 327 overlapwith each other functions as the capacitor 103. The conductive film 327is also formed over the insulating film 802 to overlap with theconductive film 324. A portion where the conductive film 324, theinsulating film 802, and the conductive film 327 overlap with each otherfunctions as the capacitor 102. The semiconductor film 333 is formedover the insulating film 802 to overlap with the conductive film 324.The wiring 23 and the conductive film 331 are formed over thesemiconductor film 333.

The insulating film 803 is formed to cover the insulating film 802 andthe conductive film 327, the semiconductor film 333, the wiring 23, andthe conductive film 331 that are formed over the insulating film 802.

Next, another example of the top view of the circuit 100 in FIG. 13A isillustrated in FIG. 79 . The top view of FIG. 79 is different from thetop view of FIG. 78 in the shape of part of the wiring 23 that overlapswith the semiconductor film 333 and the shape of part of the conductivefilm 331 that overlaps with the semiconductor film 333. Specifically, inFIG. 78 , the part of the wiring 23 that overlaps with the semiconductorfilm 333 has a U-shape. In addition, the part of the conductive film 331that overlaps with the semiconductor film 333 is positioned inside acurved portion of the U-shape of the wiring 23 to be partly surroundedby the wiring 23. In FIG. 79 , the part of the conductive film 331 thatoverlaps with the semiconductor film 333 has a U-shape. In addition, thepart of the wiring 23 that overlaps with the semiconductor film 333 ispositioned inside a curved portion of the U-shape of the conductive film331 to be partly surrounded by the conductive film 331.

In the case where the conductive film or the wiring that is in contactwith the source or the drain of the transistor 101 has a U-shape, largechannel width can be obtained even when the area of the semiconductorfilm 333 is small. Thus, on-state current can be increased while thearea of the semiconductor film 333 is small.

Note that a transistor can be formed using a variety of substrates,without limitation to a certain type. As the substrate, a semiconductorsubstrate (e.g., a single crystal substrate or a silicon substrate), anSOI substrate, a glass substrate, a quartz substrate, a plasticsubstrate, a metal substrate, a stainless steel substrate, a substrateincluding stainless steel foil, a tungsten substrate, a substrateincluding tungsten foil, a flexible substrate, an attachment film, paperincluding a fibrous material, a base material film, or the like can beused. As a glass substrate, a barium borosilicate glass substrate, analuminoborosilicate glass substrate, a soda-lime glass substrate, or thelike can be used. For a flexible substrate, a flexible synthetic resinsuch as plastics typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), or polyether sulfone (PES), or acryliccan be used. For an attachment film, polypropylene, polyester, vinyl,polyvinyl fluoride, polyvinyl chloride, or the like can be used. For abase material film, polyester, polyamide, polyimide, an inorganic vapordeposition film, paper, or the like can be used. In particular, byforming transistors with the use of a semiconductor substrate, a singlecrystal substrate, an SOI substrate, or the like, transistors with fewervariations in characteristics, sizes, shapes, or the like, with highcurrent supply capability, and with small sizes can be formed. Byforming a circuit with the use of such a transistor, the powerconsumption of the circuit can be reduced or the circuit can be highlyintegrated.

Note that the transistor may be formed using one substrate, and then,the transistor may be transferred to another substrate. In addition tothe above substrates over which the transistor can be formed, a papersubstrate, a cellophane substrate, a stone substrate, a wood substrate,a cloth substrate (including a natural fiber (e.g., silk, cotton, orhemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), or the like), a leather substrate, a rubber substrate, orthe like can be used as a substrate to which the transistor istransferred. With the use of such a substrate, a transistor withexcellent properties or a transistor with low power consumption can beformed, a device with high durability and high heat resistance can beprovided, or a reduction in weight or thickness can be achieved.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Embodiment 6

In this embodiment, specific structure examples of transistors used in asemiconductor device according to one aspect of the present inventionare described.

A transistor in FIG. 81A includes a semiconductor film 501, aninsulating film 502 over the semiconductor film 501, an electrode 503that functions as a gate and overlaps with the semiconductor film 501with the insulating film 502 provided therebetween, and conductive films504 and 505 that are in contact with the semiconductor film 501. Thesemiconductor film 501 includes a first region 506 functioning as achannel formation region and second regions 507 and 508 functioning as asource and a drain. The first region 506 is provided between the secondregions 507 and 508. Note that FIG. 81A illustrates an example in whichthe semiconductor film 501 includes third regions 509 and 510functioning as LDD regions between the first region 506 and the secondregions 507 and 508.

Note that although FIG. 81A illustrates the transistor including thethin semiconductor film 501, in one aspect of the present invention, atransistor including a channel formation region in a bulk semiconductorsubstrate may be used. For the thin semiconductor film, for example, anamorphous semiconductor, a polycrystalline semiconductor, or a singlecrystal semiconductor can be used. Further, for the semiconductor film501, a variety of semiconductors such as silicon, germanium, silicongermanium, or an oxide semiconductor can be used.

A transistor in FIG. 81B is formed on an insulating film 520 thatincludes first oxide insulating films 520 a, second oxide insulatingfilms 520 b, and a third oxide insulating film 520 c.

The first oxide insulating film 520 a and the third oxide insulatingfilm 520 c are each formed using an oxide insulating film from whichpart of oxygen is released by heating. As such an oxide insulating filmfrom which part of oxygen is released by heating, an insulating filmwhich contains oxygen at a proportion higher than the stoichiometricproportion is preferably used. Silicon oxide, silicon oxynitride,silicon nitride oxide, gallium oxide, hafnium oxide, yttrium oxide, orthe like can be used for the first oxide insulating film 520 a and thethird oxide insulating film 520 c.

The second oxide insulating film 520 b is formed using an oxideinsulating film which prevents diffusion of oxygen. The second oxideinsulating film 520 b is formed using aluminum oxide or aluminumoxynitride, for example. As aluminum oxide, aluminum oxide containingoxygen at a proportion satisfying the stoichiometric proportion oraluminum oxide containing oxygen at a proportion higher than thestoichiometric proportion (AlO_(x), where x is greater than or equal to3/2) is preferably used. In addition, in aluminum oxynitride, part ofoxygen in aluminum oxide containing oxygen at a proportion satisfyingthe stoichiometric proportion is replaced with nitrogen.

The transistor includes a semiconductor film 521, an insulating film 522over the semiconductor film 521, an electrode 523 that functions as agate and overlaps with the semiconductor film 521 with the insulatingfilm 522 provided therebetween, and conductive films 524 and 525 thatare in contact with the semiconductor film 521. The semiconductor film521 includes a first region 526 which overlaps with the electrode 523and at least part of which functions as a channel formation region, andsecond regions 550 and 551 which function as a source and a drain andsandwich the first region 526.

For the semiconductor film 521, for example, an amorphous semiconductor,a polycrystalline semiconductor, or a single crystal semiconductor canbe used. Further, for the semiconductor film 521, a variety ofsemiconductors such as silicon, germanium, silicon germanium, or anoxide semiconductor can be used.

In the transistor, sidewalls 527 including an insulating film areprovided on side portions of the electrode 523, and an insulating film528 is provided over the electrode 523. Further, part of the conductivefilm 524 and part of the conductive film 525 are in contact with thesidewalls 527. The conductive films 524 and 525 are not necessarily incontact with the sidewalls 527. However, when the conductive films 524and 525 are in contact with the sidewalls 527, the area where thesemiconductor film 521 is in contact with the conductive films 524 and525 can be prevented from changing even in the case where the conductivefilms 524 and 525 deviate from appropriate positions. Accordingly, achange in on-state current of the transistor due to deviation ofpositions of the conductive films 524 and 525 can be prevented.

Note that the insulating film 528 over the electrode 523 is notnecessarily provided. However, when the insulating film 528 is provided,conduction between the conductive film 524 and the electrode 523 andconduction between the conductive film 525 and the electrode 523 can beprevented even in the case where the conductive films 524 and 525deviate from appropriate positions and are formed over the electrode523.

In the insulating film 520, the first oxide insulating film 520 a andthe second oxide insulating film 520 b are stacked in that order overthe third oxide insulating film 520 c positioned in the lowermost layer.An opening 529 is formed in the first oxide insulating film 520 a andthe second oxide insulating film 520 b, and the semiconductor film 521of the transistor is provided in the opening 529. The first oxideinsulating film 520 a is provided around the semiconductor film 521 tobe in contact with an end portion of the semiconductor film 521. Thesecond oxide insulating film 520 b is provided around the semiconductorfilm 521 with the first oxide insulating film 520 a providedtherebetween. The third oxide insulating film 520 c is provided belowthe semiconductor film 521.

In the case where the semiconductor film 521 is formed using an oxidesemiconductor, the use of the insulating film 520 with the abovestructure can prevent oxygen released from the first oxide insulatingfilm 520 a by heating from passing through the second oxide insulatingfilm 520 b; thus, oxygen is efficiently supplied to end portions of thesemiconductor film 521 in the first region 526. Further, oxygen releasedfrom the third oxide insulating film 520 c is supplied to a lowerportion of the semiconductor film 521. Note that oxygen vacancies due torelease of oxygen easily occur at an end portion of the semiconductorfilm 521 in a transistor in which an oxide semiconductor is used for achannel formation region because of etching treatment for etching thesemiconductor film 521 into a desired shape, exposure of the end portionof the semiconductor film 521 to a reduced-pressure atmosphere, or thelike. Since oxygen vacancies become a path through which carriers move,a parasitic channel is formed when oxygen vacancies occur at the endportion of the semiconductor film 521. Consequently, the off-statecurrent of the transistor is increased. However, with the abovestructure, oxygen vacancies are prevented from occurring at the endportion of the semiconductor film 521 in the first region 526.Consequently, the off-state current can be decreased.

Note that “to release part of oxygen by heating” means that the amountof released oxygen is greater than or equal to 1.0×10¹⁸ atoms/cm³,preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in thermaldesorption spectroscopy (TDS) analysis on an oxygen atom basis.

A method for measuring the amount of released oxygen on an oxygen atombasis in TDS analysis is described below.

The amount of a released gas in the TDS analysis is proportional to theintegral value of a spectrum. Thus, the amount of a released gas can becalculated from the ratio between the integral value of a spectrum of aninsulating film and the reference value of a standard sample. Thereference value of a standard sample is the ratio of the density of apredetermined atom contained in a sample to the integral value of aspectrum.

For example, the amount of released oxygen molecules (N_(O2)) from aninsulating film can be calculated from Formula 1 with the TDS analysisresults of a silicon wafer containing hydrogen at predetermined densitythat is the standard sample and the TDS analysis results of theinsulating film CH₃OH, which is a gas having a mass number of 32, isunlikely to be present in the insulating film. Thus, all spectra havinga mass number of 32 that are obtained by the TDS analysis are assumed tooriginate from an oxygen molecule. Further, an oxygen molecule includingan oxygen atom having a mass number of 17 or 18 that is an isotope of anoxygen atom is assumed not to be present because the proportion of sucha molecule in the natural world is minimal.N _(O2) =N _(H2) /S _(H2) ×S _(O2)×α  (Formula 1)

N_(H2) is the value obtained by conversion of the amount of hydrogenmolecules released from the standard sample into density. S_(H2) is theintegral value of a spectrum of the standard sample which is analyzed byTDS. The reference value of the standard sample is set to N_(H2)/S_(H2).S_(O2) is the integral value of a spectrum of the insulating film whichis analyzed by TDS. α is a coefficient influencing the intensity of thespectrum in the TDS analysis. Refer to Japanese Published PatentApplication No. 6-275697 for details of Formula 1. Note that the amountof released oxygen from the insulating film is measured with a thermaldesorption spectroscopy apparatus produced by ESCO Ltd., EMD-WA1000S/W,using a silicon wafer containing hydrogen atoms at 1×10¹⁶ atoms/cm³ asthe standard sample.

Further, in the TDS analysis, part of oxygen is detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note thatsince α includes the ionization rate of oxygen molecules, the amount ofreleased oxygen atoms can also be estimated through evaluation of theamount of released oxygen molecules.

Note that N_(O2) is the amount of released oxygen molecules. For theinsulating film, the amount of released oxygen on an oxygen atom basisis twice the amount of released oxygen molecules.

In the above structure, the insulating film from which oxygen isreleased by heating may be oxygen-excess silicon oxide (SiO_(X) (X>2)).In oxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygenatoms per unit volume is more than twice the number of silicon atoms perunit volume. The number of silicon atoms and the number of oxygen atomsper unit volume are measured by Rutherford backscattering spectrometry.

A transistor in FIG. 81C is formed on an insulating film 530 thatincludes a first oxide insulating film 530 a and second oxide insulatingfilms 530 b.

The first oxide insulating film 530 a is formed using an oxideinsulating film from which part of oxygen is released by heating. Assuch an oxide insulating film from which part of oxygen is released byheating, an insulating film which contains oxygen at a proportion higherthan the stoichiometric proportion is preferably used. Silicon oxide,silicon oxynitride, silicon nitride oxide, gallium oxide, hafnium oxide,yttrium oxide, or the like can be used for the first oxide insulatingfilm 530 a.

The second oxide insulating film 530 b is formed using an oxideinsulating film which prevents diffusion of oxygen. The second oxideinsulating film 530 b is formed using aluminum oxide or aluminumoxynitride, for example. As aluminum oxide, aluminum oxide containingoxygen at a proportion satisfying the stoichiometric proportion oraluminum oxide containing oxygen at a proportion higher than thestoichiometric proportion (AlO_(x), where x is greater than or equal to3/2) is preferably used. In addition, in aluminum oxynitride, part ofoxygen in aluminum oxide containing oxygen at a proportion satisfyingthe stoichiometric proportion is replaced with nitrogen.

The transistor includes a semiconductor film 531 over the insulatingfilm 530, an insulating film 532 over the semiconductor film 531, anelectrode 533 that functions as a gate and overlaps with thesemiconductor film 531 with the insulating film 532 providedtherebetween, and conductive films 534 and 535 that are connected to thesemiconductor film 531. The semiconductor film 531 includes a firstregion 536 which overlaps with the electrode 533 and at least part ofwhich functions as a channel formation region, and second regions 537and 538 which function as a source and a drain and sandwich the firstregion 536.

For the semiconductor film 531, for example, an amorphous semiconductor,a polycrystalline semiconductor, or a single crystal semiconductor canbe used. Further, for the semiconductor film 531, a variety ofsemiconductors such as silicon, germanium, silicon germanium, or anoxide semiconductor can be used.

In the transistor, sidewalls 539 including an insulating film areprovided on side portions of the electrode 533, and an insulating film540 is provided over the electrode 533. Further, part of the conductivefilm 534 and part of the conductive film 535 are in contact with thesidewalls 539. The conductive films 534 and 535 are not necessarily incontact with the sidewalls 539. However, when the conductive films 534and 535 are in contact with the sidewalls 539, the area where thesemiconductor film 531 is in contact with the conductive films 534 and535 can be prevented from changing even in the case where the conductivefilms 534 and 535 deviate from appropriate positions. Accordingly, achange in on-state current of the transistor due to deviation ofpositions of the conductive films 534 and 535 can be prevented.

Note that the insulating film 540 over the electrode 533 is notnecessarily provided. However, when the insulating film 540 is provided,conduction between the conductive film 534 and the electrode 533 andconduction between the conductive film 535 and the electrode 533 can beprevented even in the case where the conductive films 534 and 535deviate from appropriate positions and are formed over the electrode533.

Further, in the insulating film 530, the second oxide insulating film530 b is provided around the first oxide insulating film 530 a. Thefirst region 536 of the semiconductor film 531 is in contact with thefirst oxide insulating film 530 a, and the second regions 537 and 538 ofthe semiconductor film 531 are in contact with the first oxideinsulating film 530 a and the second oxide insulating film 530 b.

In the case where the semiconductor film 531 is formed using an oxidesemiconductor, the above structure can prevent oxygen released from thefirst oxide insulating film 530 a by heating from passing through thesecond oxide insulating film 530 b; thus, oxygen is efficiently suppliedto end portions of the semiconductor film 531 in the first region 536.Note that oxygen vacancies due to release of oxygen easily occur at anend portion of the semiconductor film 531 in a transistor in which anoxide semiconductor is used for a channel formation region because ofetching treatment for etching the semiconductor film 531 into a desiredshape, exposure of the end portion of the semiconductor film 531 to areduced-pressure atmosphere, or the like. Since oxygen vacancies becomea path through which carriers move, a parasitic channel is formed whenoxygen vacancies occur at the end portion of the semiconductor film 531.Consequently, the off-state current of the transistor is increased.However, in one aspect of the present invention, with the abovestructure, oxygen vacancies are prevented from being occurring at theend portion of the semiconductor film 531 in the first region 536.Consequently, the off-state current can be decreased.

Note that a highly-purified oxide semiconductor (a purified oxidesemiconductor) obtained by a reduction of impurities such as moisture orhydrogen that serve as electron donors (donors) and a reduction ofoxygen vacancies is an intrinsic (i-type) semiconductor or asubstantially intrinsic semiconductor. Thus, a transistor including theoxide semiconductor has extremely low off-state current. Further, theband gap of the oxide semiconductor is 2 eV or more, preferably 2.5 eVor more, more preferably 3 eV or more. With the use of an oxidesemiconductor film that is highly purified by a sufficient decrease inconcentration of impurities such as moisture or hydrogen and a reductionof oxygen vacancies, the off-state current of the transistor can bedecreased.

Specifically, various experiments can prove low off-state current of atransistor including a highly-purified oxide semiconductor film for achannel formation region. For example, even when an element has achannel width of 1×10⁶ μm and a channel length of 10 μm, off-statecurrent can be lower than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., lower than or equal to 1×10⁻¹³A, at a voltage (drain voltage) between a source electrode and a drainelectrode of 1 to 10 V. In that case, it can be seen that off-statecurrent standardized on the channel width of the transistor is lowerthan or equal to 100 zA/μm. In addition, a capacitor and a transistorwere connected to each other and off-state current was measured using acircuit in which electrical charge flowing to or from the capacitor iscontrolled by the transistor. In the measurement, a highly-purifiedoxide semiconductor film was used for a channel formation region of thetransistor, and the off-state current of the transistor was measuredfrom a change in the amount of electrical charge of the capacitor perunit hour. As a result, it can be seen that, in the case where thevoltage between the source electrode and the drain electrode of thetransistor is 3 V, a lower off-state current of several tens ofyoctoamperes per micrometer (yA/μm) is obtained. Accordingly, thetransistor including the highly-purified oxide semiconductor film for achannel formation region has much lower off-state current than acrystalline silicon transistor.

Note that as the oxide semiconductor, preferably an oxide containing Inor Zn, more preferably an oxide containing In and Ga or an oxidecontaining In and Zn is used. In order to obtain an intrinsic (i-type)oxide semiconductor film, dehydration or dehydrogenation to be describedlater is effective. As a stabilizer for reducing variations inelectrical characteristics of a transistor including the oxidesemiconductor, gallium (Ga) is preferably additionally contained. Tin(Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferablycontained as a stabilizer. Aluminum (Al) is preferably contained as astabilizer. Zirconium (Zr) is preferably contained as a stabilizer.

As another stabilizer, one or more kinds of lanthanoid such as lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

For example, indium oxide; tin oxide; zinc oxide; a binary metal oxidesuch as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide,a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; a ternary metal oxide such as an In—Ga—Zn-based oxide(also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-basedoxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or a quaternarymetal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used as anoxide semiconductor. The oxide semiconductor may contain silicon.

Note that, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn, and there is no limitation on the ratio ofIn, Ga, and Zn. In addition, the In—Ga—Zn-based oxide may contain ametal element other than In, Ga, and Zn. The In—Ga—Zn-based oxide hassufficiently high resistance when no electric field is applied thereto,so that off-state current can be sufficiently reduced. Further, theIn—Ga—Zn-based oxide has high mobility.

An oxide semiconductor film can be single crystal, polycrystalline (alsoreferred to as polycrystal), or amorphous, for example. The oxidesemiconductor film is preferably a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where a crystal part with a sizeof several nanometers to several tens of nanometers is included in anamorphous phase. Note that with a transmission electron microscope(TEM), a boundary between an amorphous part and a crystal part in theCAAC-OS film is not clear. Further, a grain boundary is not found in theCAAC-OS film. Since the CAAC-OS film does not have a grain boundary, adecrease in electron mobility due to a grain boundary does not easilyoccur.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic order which is seen fromthe direction perpendicular to the a-b plane is formed, and metal atomsare arranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that, among crystal parts, the directions of thea-axis and the b-axis of one crystal part may be different from those ofanother crystal part. In this specification, a simple term“perpendicular” includes a range from 85 to 95°. In addition, a simpleterm “parallel” includes a range from −5 to 5°.

Note that the proportions of the amorphous parts and the crystal partsin the CAAC-OS film are not necessarily uniform. For example, in thecase where crystal growth occurs from a surface side of the CAAC-OSfilm, in some cases, the proportion of the crystal parts in the vicinityof the surface of the CAAC-OS is high and the proportion of theamorphous parts in the vicinity of the surface where the CAAC-OS film isformed is high. Further, when an impurity is added to the CAAC-OS film,the crystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to the normal vector of the surfacewhere the CAAC-OS film is formed or the normal vector of the surface ofthe CAAC-OS film, the directions of the c-axes of the crystal parts maybe different from each other depending on the shape of the CAAC-OS film(the cross-sectional shape of the surface where the CAAC-OS film isformed or the cross-sectional shape of the surface of the CAAC-OS film).Note that when the CAAC-OS film is formed, the direction of the c-axisof the crystal part is the direction parallel to the normal vector ofthe surface where the CAAC-OS film is formed or the normal vector of thesurface of the CAAC-OS film. The crystal part is formed by deposition orby performing treatment for crystallization such as heat treatment afterdeposition.

With the use of the CAAC-OS film, a change in electrical characteristicsof the transistor due to irradiation with visible light or ultravioletlight can be reduced, so that a highly reliable transistor can beobtained.

For example, a CAAC-OS film is deposited by sputtering with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along the a-b plane, and asputtered particle having a plane parallel to the a-b plane (aflat-plate-like sputtered particle or a pellet-like sputtered particle)might be separated from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingits crystal state, so that the CAAC-OS film can be deposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably employed.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thetreatment chamber may be reduced. Further, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle occurs after the sputtered particlereaches the substrate. Specifically, the substrate heating temperatureduring the deposition is 100° C. or higher and 740° C. or lower,preferably 200° C. or higher and 500° C. or lower. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate, so that a flat plane of the sputtered particleis attached to the substrate.

Further, it is preferable to reduce plasma damage during the depositionby increasing the proportion of oxygen in the deposition gas andoptimizing power. The proportion of oxygen in the deposition gas is 30vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

A polycrystalline In—Ga—Zn—O compound target is made by mixing InO_(X)powder, GaO_(Y) powder, and ZnO_(Z) powder in a predetermined moleratio, applying pressure, and performing heat treatment at 1000° C. orhigher and 1500° C. or lower. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined mole ratio of the InO_(X)powder, the GaO_(Y) powder, and the ZnO_(Z) powder is, for example,2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and themole ratio for mixing powder may be changed as appropriate depending ona sputtering target to be formed.

For example, the oxide semiconductor film can be formed by sputteringusing a target including indium (In), gallium (Ga), and zinc (Zn). Inthe case where an In—Ga—Zn-based oxide semiconductor film is formed bysputtering, it is preferable to use a target of an In—Ga—Zn-based oxidewith an atomic ratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or3:1:4. When an oxide semiconductor film is formed using a target of anIn—Ga—Zn-based oxide having the above atomic ratio, a polycrystal orCAAC-OS is easily formed. The relative density of the target includingIn, Ga, and Zn is higher than or equal to 90% and lower than or equal to100%, preferably higher than or equal to 95% and lower than 100%. Withthe use of the target with high relative density, a dense oxidesemiconductor film is formed.

In the case where an In—Zn-based material is used for the oxidesemiconductor, a target used has an atomic ratio of In:Zn=50:1 to 1:2(In₂O₃:ZnO=25:1 to 1:4 in a mole ratio), preferably In:Zn=20:1 to 1:1(In₂O₃:ZnO=10:1 to 1:2 in a mole ratio), more preferably In:Zn=15:1 to1.5:1 (In₂O₃:ZnO=15:2 to 3:4 in a mole ratio). For example, when atarget used for deposition of an oxide semiconductor film formed usingan In—Zn-based oxide has an atomic ratio of In:Zn:O=X:Y:Z, Z>1.5X+Y. Themobility can be increased by keeping the ratio of Zn within the aboverange.

Specifically, the oxide semiconductor film may be deposited in such amanner that the substrate is held in a treatment chamber kept in areduced pressure state, moisture remaining in the treatment chamber isremoved, a sputtering gas from which hydrogen and moisture are removedis introduced, and the target is used. The substrate temperature may be100 to 600° C., preferably 200 to 400° C. during deposition. Bydeposition of the oxide semiconductor film while the substrate isheated, the concentration of impurities included in the deposited oxidesemiconductor film can be lowered. In addition, damage by sputtering canbe reduced. In order to remove moisture remaining in the treatmentchamber, an adsorption vacuum pump is preferably used. For example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. A turbo pump to which a cold trap is added may be used as anexhaustion means. For example, a hydrogen atom, a compound containing ahydrogen atom, such as water (preferably a compound containing a carbonatom), and the like are exhausted from the treatment chamber with theuse of a cryopump. Thus, the concentration of impurities contained inthe oxide semiconductor film deposited in the treatment chamber can belowered.

Note that the oxide semiconductor film formed by sputtering or the likecontains a large amount of moisture or hydrogen (including a hydroxylgroup) as an impurity in some cases. Moisture and hydrogen easily formdonor levels and thus serve as impurities in the oxide semiconductor.Thus, in one embodiment of the present invention, in order to reduceimpurities such as moisture or hydrogen in the oxide semiconductor film(in order to perform dehydration or dehydrogenation), the oxidesemiconductor film is subjected to heat treatment in a reduced-pressureatmosphere, an inert gas atmosphere of nitrogen, a rare gas, or thelike, an oxygen gas atmosphere, or ultra dry air (the moisture amount is20 ppm (−55° C. by conversion into a dew point) or less, preferably 1ppm or less, more preferably 10 ppb or less, in the case wheremeasurement is performed by a dew point meter in a cavity ring-downlaser spectroscopy (CRDS) method).

By performing heat treatment on the oxide semiconductor film, moistureor hydrogen in the oxide semiconductor film can be eliminated.Specifically, heat treatment may be performed at a temperature higherthan or equal to 250° C. and lower than or equal to 750° C., preferablyhigher than or equal to 400° C. and lower than the strain point of thesubstrate. For example, heat treatment may be performed at 500° C. forapproximately 3 to 6 minutes. When RTA is used for the heat treatment,dehydration or dehydrogenation can be performed in a short time; thus,treatment can be performed even at a temperature higher than the strainpoint of a glass substrate.

Note that in some cases, the heat treatment makes oxygen released fromthe oxide semiconductor film and oxygen vacancies occur in the oxidesemiconductor film Thus, in one embodiment of the present invention, aninsulating film containing oxygen is used as an insulating film that isin contact with the oxide semiconductor film, such as a gate insulatingfilm. Then, heat treatment is performed after formation of theinsulating film containing oxygen, so that oxygen is supplied from theinsulating film to the oxide semiconductor film With this structure,oxygen vacancies that serve as donors can be reduced and thestoichiometric proportion of the oxide semiconductor included in theoxide semiconductor film can be satisfied. It is preferable that theproportion of oxygen in the oxide semiconductor film be higher than thestoichiometric proportion. As a result, the oxide semiconductor film canbe substantially intrinsic and variations in electrical characteristicsof the transistor due to oxygen vacancies can be reduced, which resultsin an improvement of electrical characteristics.

Note that the heat treatment for supplying oxygen to the oxidesemiconductor film is performed in an atmosphere of nitrogen, ultra dryair, or a rare gas (e.g., argon or helium) preferably at 200 to 400° C.,for example, 250 to 350° C. It is preferable that the water content inthe gas be 20 ppm or less, preferably 1 ppm or less, more preferably 10ppb or less.

A transistor illustrated in FIG. 82A is a bottom-gate transistor with achannel-etched structure.

The transistor illustrated in FIG. 82A includes a gate electrode 602formed over an insulating surface, a gate insulating film 603 over thegate electrode 602, a semiconductor film 604 over the gate insulatingfilm 603 that overlaps with the gate electrode 602, and conductive films605 and 606 formed over the semiconductor film 604. The transistor mayfurther include an insulating film 607 formed over the semiconductorfilm 604 and the conductive films 605 and 606.

Note that the transistor illustrated in FIG. 82A may further include aback-gate electrode formed over the insulating film 607 in a portionthat overlaps with the semiconductor film 604.

A transistor illustrated in FIG. 82B is a bottom-gate transistor with achannel-protective structure.

The transistor illustrated in FIG. 82B includes a gate electrode 612formed over an insulating surface, a gate insulating film 613 over thegate electrode 612, a semiconductor film 614 over the gate insulatingfilm 613 that overlaps with the gate electrode 612, a channel protectivefilm 618 formed over the semiconductor film 614, and conductive films615 and 616 formed over the semiconductor film 614. The transistor mayfurther include an insulating film 617 formed over the channelprotective film 618 and the conductive films 615 and 616.

Note that the transistor illustrated in FIG. 82B may further include aback-gate electrode formed over the insulating film 617 in a portionthat overlaps with the semiconductor film 614.

The channel protective film 618 can prevent a portion of thesemiconductor film 614 that serves as a channel formation region frombeing damaged in a later step, for example, a reduction in thickness dueto plasma or an etchant during etching. Thus, the reliability of thetransistor can be improved.

A transistor illustrated in FIG. 82C is a bottom-gate transistor with abottom-contact structure.

The transistor illustrated in FIG. 82C includes a gate electrode 622formed over an insulating surface, a gate insulating film 623 over thegate electrode 622, conductive films 625 and 626 over the gateinsulating film 623, and a semiconductor film 624 over the gateinsulating film 623 that overlaps with the gate electrode 622 and isformed over the conductive films 625 and 626. Further, the transistormay include an insulating film 627 formed over the conductive films 625and 626 and the semiconductor film 624.

Note that the transistor illustrated in FIG. 82C may further include aback-gate electrode formed over the insulating film 627 in a portionthat overlaps with the semiconductor film 624.

A transistor illustrated in FIG. 82D is a top-gate transistor with abottom-contact structure.

The transistor illustrated in FIG. 82D includes conductive films 645 and646 formed over an insulating surface, a semiconductor film 644 formedover the conductive films 645 and 646, a gate insulating film 643 formedover the semiconductor film 644 and the conductive films 645 and 646,and a gate electrode 642 over the gate insulating film 643 that overlapswith the semiconductor film 644. Further, the transistor may include aninsulating film 647 formed over the gate electrode 642.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Embodiment 7

In this embodiment, a light-emitting device that is one aspect of asemiconductor device of the present invention is given as an example,and the appearance of the light-emitting device is described withreference to FIGS. 83A and 83B. FIG. 83A is a top view of a panel inwhich a transistor and a light-emitting element that are formed over afirst substrate are sealed between the first substrate and a secondsubstrate with a sealant. FIG. 83B corresponds to a cross-sectional viewtaken along line A-A′ in FIG. 83A.

A sealant 4020 is formed to surround a pixel area 4002, a circuit 4003,and a circuit 4004 that are provided over a first substrate 4001. Inaddition, a second substrate 4006 is formed over the pixel area 4002,the circuit 4003, and the circuit 4004. Thus, the pixel area 4002, thecircuit 4003, and the circuit 4004 are sealed together with a filler4007 between the first substrate 4001 and the second substrate 4006 withthe sealant 4020.

Each of the pixel area 4002 and the circuits 4003 and 4004 for supplyingsignals to the pixel area 4002 is formed over the first substrate 4001and has a plurality of transistors. In FIG. 83B, a transistor 4008included in the circuit 4003, and transistors 4009 and 4010 included inthe pixel area 4002 are illustrated.

In addition, part of a wiring 4017 which is connected to a source or adrain of the transistor 4009 is used as a pixel electrode of alight-emitting element 4011. Further, the light-emitting element 4011includes a counter electrode 4012 and a light-emitting layer 4013 inaddition to the pixel electrode. Note that the structure of thelight-emitting element 4011 is not limited to the structure described inthis embodiment. The structure of the light-emitting element 4011 can bechanged as appropriate depending on the direction of light extractedfrom the light-emitting element 4011, the polarity of the transistor4009, or the like.

Although a variety of signals and voltage supplied to the circuit 4003,the circuit 4004, or the pixel area 4002 are not illustrated in thecross-sectional view of FIG. 83B, the variety of signals and voltage aresupplied from a connection terminal 4016 through lead wirings 4014 and4015.

In this embodiment, the connection terminal 4016 is formed using thesame conductive film as the counter electrode 4012 of the light-emittingelement 4011. The lead wiring 4014 is formed using the same conductivefilm as the wiring 4017. Further, the lead wiring 4015 is formed usingthe same conductive film as gate electrodes of the transistors 4009,4010, and 4008.

The connection terminal 4016 is electrically connected to a terminal ofan FPC 4018 through an anisotropic conductive film 4019.

The first substrate 4001 and the second substrate 4006 can be formedusing glass, metal (typically, stainless steel), ceramics, or plastics.Note that the second substrate 4006 positioned in a direction from whichlight from the light-emitting element 4011 is extracted should have alight-transmitting property. Thus, a light-transmitting material such asa glass plate, a plastic plate, a polyester film, or an acrylic film ispreferably used for the second substrate 4006.

In addition, an ultraviolet curable resin or a thermosetting resin aswell as an inert gas such as nitrogen or argon can be used for thefiller 4007. In this embodiment, an example in which nitrogen is usedfor the filler 4007 is illustrated.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Embodiment 8

The circuit 100 according to one aspect of the present invention can beused for a pixel area of a display device. Alternatively, the circuit100 according to one aspect of the present invention can be used for adriver circuit of a display device.

FIG. 84A is a block diagram of a display device that is a semiconductordevice according to one aspect of the present invention. The displaydevice in FIG. 84A includes a pixel area 700, a driver circuit 701, anda driver circuit 702. The pixel area 700 includes the plurality ofcircuits 100 functioning as pixels. The driver circuit 701 and thedriver circuit 702 have a function of supplying a variety of constantvoltage or signals to the circuits 100.

FIG. 84B is a block diagram of a display device that is a semiconductordevice according to one aspect of the present invention. The displaydevice in FIG. 84B includes a pixel area 711 and a driver circuit 710.The driver circuit 710 includes the plurality of circuits 100functioning as current sources. Current output from the circuit 100 issupplied to a pixel included in the pixel area 711.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Embodiment 9

A semiconductor device according to one embodiment of the presentinvention can be used for display devices, personal computers, or imagereproducing devices provided with recording media (typically, devicesthat reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Further, as electronic devices that can include the semiconductor deviceaccording to one embodiment of the present invention, cellular phones,game machines (including portable game machines), personal digitalassistants, e-book readers, cameras such as video cameras and digitalstill cameras, goggle-type displays (head mounted displays), navigationsystems, audio reproducing devices (e.g., car audio systems and digitalaudio players), copiers, facsimiles, printers, multifunction printers,automated teller machines (ATMs), vending machines, and the like can begiven. FIGS. 85A to 85F and FIG. 91 illustrate specific examples ofthese electronic devices.

FIG. 85A illustrates a portable game machine, which includes a housing5001, a housing 5002, a display portion 5003, a display portion 5004, amicrophone 5005, speakers 5006, a control key 5007, a stylus 5008, andthe like. The semiconductor device according to one aspect of thepresent invention can be used for an integrated circuit for controllingdriving of the portable game machine or the display portion 5003 or5004. Note that although the portable game machine in FIG. 85A has thetwo display portions 5003 and 5004, the number of display portionsincluded in the portable game machine is not limited thereto.

FIG. 85B illustrates a display device, which includes a housing 5201, adisplay portion 5202, a support 5203, and the like. The semiconductordevice according to one aspect of the present invention can be used foran integrated circuit for controlling driving of the display device orthe display portion 5202. Note that the display device means all displaydevices for displaying information, such as display devices for personalcomputers, for receiving TV broadcast, and for displayingadvertisements.

FIG. 85C illustrates a laptop, which includes a housing 5401, a displayportion 5402, a keyboard 5403, a pointing device 5404, and the like. Thesemiconductor device according to one aspect of the present inventioncan be used for an integrated circuit for controlling driving of thelaptop or the display portion 5402.

FIG. 85D illustrates a personal digital assistant, which includes ahousing 5601, a display portion 5602, control keys 5603, and the like.In the personal digital assistant in FIG. 85D, a modem may beincorporated in the housing 5601. The semiconductor device according toone aspect of the present invention can be used for an integratedcircuit for controlling driving of the personal digital assistant or thedisplay portion 5602.

FIG. 85E illustrates a cellular phone, which includes a housing 5801, adisplay portion 5802, an audio input portion 5803, an audio outputportion 5804, control keys 5805, a light-receiving portion 5806, and thelike. Light received in the light-receiving portion 5806 is convertedinto electrical signals, so that external images can be loaded. Thesemiconductor device according to one aspect of the present inventioncan be used for an integrated circuit for controlling driving of thecellular phone or the display portion 5802.

FIG. 85F illustrates a personal digital assistant, which includes afirst housing 5901, a second housing 5902, a first display portion 5903,a second display portion 5904, a joint 5905, a control key 5906, and thelike. The first display portion 5903 is provided in the first housing5901, and the second display portion 5904 is provided in the secondhousing 5902. The first housing 5901 and the second housing 5902 areconnected to each other with the joint 5905, and an angle between thefirst housing 5901 and the second housing 5902 can be changed with thejoint 5905. An image on the first display portion 5903 may be switcheddepending on the angle between the first housing 5901 and the secondhousing 5902 at the joint 5905. The semiconductor device according toone aspect of the present invention can be used for an integratedcircuit for controlling driving of the personal digital assistant, thefirst display portion 5903, or the second display portion 5904. Adisplay device with a position input function may be used as at leastone of the first display portion 5903 and the second display portion5904. Note that the position input function can be added by provision ofa touch panel in a display device. Alternatively, the position inputfunction can be added by provision of a photoelectric conversion elementcalled a photosensor in a pixel area of a display device.

Next, a structure example of a cellular phone according to the presentinvention is described with reference to FIG. 91 .

A display panel 900501 is detachably incorporated in a housing 900530.The shape or size of the housing 900530 can be changed as appropriatedepending on the size of the display panel 900501. The housing 900530 towhich the display panel 900501 is fixed is fitted in a printed board900531 to be assembled as a module.

By provision of a touch panel, an FPC, a printed board, a frame, aradiator plate, an optical film, a polarizing plate, a retardation film,a prism sheet, a diffusion plate, a backlight, a light guide plate, anLED, a CFL, a front light, a controller, a driver circuit, a signalprocessing circuit, or the like, the display panel 900501 can be used asa display module. Further, a counter substrate (a sealing substrate) ofthe display panel 900501 can function as a touch panel.

The display panel 900501 is connected to the printed board 900531through an FPC 900513. The printed board 900531 is provided with aspeaker 900532, a microphone 900533, a transmission/reception circuit900534, and a signal processing circuit 900535 including a CPU, acontroller, and the like. Such a module, an input means 900536, and abattery 900537 are combined and stored in a housing 900539. A pixel areaof the display panel 900501 is provided to be seen from an openingwindow formed in the housing 900539.

In the display panel 900501, the pixel area and part of peripheraldriver circuits (a driver circuit having low operation frequency among aplurality of driver circuits) may be formed over one substrate by usingTFTs, and another part of the peripheral driver circuits (a drivercircuit having high operation frequency among the plurality of drivercircuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 900501 by chip on glass (COG).Alternatively, the IC chip may be connected to a glass substrate byusing tape automated bonding (TAB) or a printed board. With such astructure, the power consumption of a display device can be reduced, andthe operation time of the cellular phone per charge can be extended.Further, the cost of the cellular phone can be reduced.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

Note that in a diagram or a text described in one embodiment, part ofthe diagram or the text is taken out, and one embodiment of theinvention can be constituted. Thus, in the case where a diagram or atext related to a certain portion is described, the context taken outfrom part of the diagram or the text is also disclosed as one embodimentof the invention, and one embodiment of the invention can beconstituted. Therefore, for example, in a diagram or a text in which oneor more active elements (e.g., transistors or diodes), wirings, passiveelements (e.g., capacitors or resistors), conductive layers, insulatinglayers, semiconductor layers, organic materials, inorganic materials,components, devices, operating methods, manufacturing methods, or thelike are described, part of the diagram or the text is taken out, andone embodiment of the invention can be constituted. For example, Mcircuit elements (e.g., transistors or capacitors) (M is an integer,where M<N) are taken out from a circuit diagram in which N circuitelements (e.g., transistors or capacitors) (N is an integer) areprovided, and one embodiment of the invention can be constituted. Asanother example, M layers (M is an integer, where M<N) are taken outfrom a cross-sectional view in which N layers (N is an integer) areprovided, and one embodiment of the invention can be constituted. Asanother example, M elements (M is an integer, where M<N) are taken outfrom a flow chart in which N elements (N is an integer) are provided,and one embodiment of the invention can be constituted.

Note that in a diagram or a text described in one embodiment, in thecase where at least one specific example is described, it will bereadily appreciated by those skilled in the art that a broader conceptof the specific example can be derived. Thus, in the diagram or the textdescribed in one embodiment, in the case where at least one specificexample is described, a broader concept of the specific example isdisclosed as one embodiment of the invention, and one embodiment of theinvention can be constituted.

Note that a content described in at least a diagram (or may be part ofthe diagram) is disclosed as one embodiment of the invention, and oneembodiment of the invention can be constituted. Thus, when a certaincontent is described in a diagram, the content is disclosed as oneembodiment of the invention even when the content is not described witha text, and one embodiment of the invention can be constituted.Similarly, part of a diagram that is taken out from the diagram isdisclosed as one embodiment of the invention, and one embodiment of theinvention can be constituted.

Embodiment 10

FIG. 88A illustrates a structure example of the circuit 100, in whichtransistors are used as the switch 11, the switch 12, the switch 13, theswitch 14, and the switch 914 in FIG. 49A.

In the circuit 100 in FIG. 88A, the transistor 11 t, the transistor 12t, the transistor 13 t, the transistor 14 t, and the transistor 914 tare used as the switch 11, the switch 12, the switch 13, the switch 14,and the switch 914, respectively.

FIG. 88A illustrates the example in which the transistor 11 t, thetransistor 12 t, the transistor 13 t, the transistor 14 t, and thetransistor 914 t are all n-channel transistors. When the transistor 11t, the transistor 12 t, the transistor 13 t, the transistor 14 t, andthe transistor 914 t have the same polarity, these transistors can bemanufactured in a small number of steps. However, one aspect of theembodiment of the present invention is not limited thereto, and thesetransistors can have different polarities.

In FIG. 88A, the gate of the transistor 11 t is connected to the wiring31. The transistor 11 t is turned on or off in response to a potentialsupplied to the wiring 31. The gate of the transistor 12 t is connectedto the wiring 32. The transistor 12 t is turned on or off in response toa potential supplied to the wiring 32. The gate of the transistor 13 tis connected to the wiring 32. The transistor 13 t is turned on or offin response to the potential supplied to the wiring 32. The gate of thetransistor 14 t is connected to the wiring 34. The transistor 14 t isturned on or off in response to a potential supplied to the wiring 34.Thus, it is preferable that the potentials of the wirings 31, 32, and 34be pulsed potentials and not constant; however, one aspect of theembodiment of the present invention is not limited thereto.Alternatively, the wirings 31, 32, and 34 each function as a gate signalline, a selection signal line, or a scan line.

Note that in FIG. 88A, the gate of the transistor 12 t and the gate ofthe transistor 13 t are connected to the wiring 32. In one aspect of thepresent invention, the gate of the transistor 12 t may be connected tothe wiring 32, and the gate of the transistor 13 t may be connected tothe wiring 33.

At least two of the wirings 31, 32, and 34 can be connected to eachother. Alternatively, at least one of the wirings 31, 32, and 34 can beconnected to at least one of the wirings 31, 32, and 34 in anothercircuit 100.

The semiconductor device according to one aspect of the presentinvention may further include a circuit for supplying a variety ofconstant voltage or signals to the circuit 100, in addition to thecircuit 100 in FIG. 88A.

The value of the gate-source voltage Vgs101 of the transistor 101 in theperiod T14 during which the light-emitting element 104 a emits light atluminance based on the potential Vsig was calculated. The circuit 100 inFIG. 88A was used in the calculation.

FIG. 88B is a timing chart illustrating the potentials of the wiringsincluded in the circuit 100 in FIG. 88A in the calculation.Specifically, FIG. 88B illustrates time changes of the potential of thewiring 21, the potential of the wiring 34, the potential of the wiring32, the potential of the wiring 31, and the potential of the wiring 932.Note that in FIG. 88B, a high potential GVDD or a low potential GVSS isapplied to the wiring 34, the wiring 32, the wiring 31, and the wiring932.

The calculation was performed under Condition A and Condition B withdifferent values of the potential Vi2 of the wiring 22. Table 1 showsspecific potentials of the wirings under Condition A and Condition B.Note that in Table 1, the potential Vcat of the wiring 24 is 0 V, andthe values of the potential Vsig, the potential Vi1, the potential VDD,the potential Vi2, the potential GVDD, and the potential GVSS arerepresented by a potential difference with respect to the potentialVcat.

TABLE 1 Condition A Condition B Vth −3 V to 3 V −3 V to 3 V Vsig  4 V to9 V  4 V to 9 V Vi1 4 V 4 V VDD 14 V 14 V Vi2 8 V 14 V Vcat 0 V 0 VGVDD/GVSS 17 V/−5 V 20 V/−5 V

As for the channel length L to channel width W ratio of the transistorsin the calculation, L/W of the transistor 101 was 10 μm/10 μm, L/W ofthe transistors 12 t and 13 t were each 6 μm/5 μm, and L/W of thetransistors 11 t, 14 t, and 914 t were each 6 μm/9 μm. Assuming that aregion A is a region where the semiconductor film is in contact with thesource electrode or the drain electrode in all the transistors includedin the circuit 100 in FIG. 88A, the length (Lov) in the channel lengthdirection of a region where the region A overlaps with the gateelectrode was 2.0 μm.

In the period T14 in FIG. 88B, the gate-source voltage Vgs101 of thetransistor 101 becomes the voltage Vsig+Vth−Vi1. Thus, Vgs101−Vth equalsVsig−Vi1 in the circuit 100 in FIG. 88A, so that Vgs101−Vth is ideallyconstant regardless of the value of the threshold voltage Vth.

FIG. 89 shows the value of Vgs101−Vth obtained by the calculation underCondition A. In FIG. 89 , the horizontal axis represents the value ofthe threshold voltage Vth (V) and the vertical axis represents the valueof Vgs101−Vth (V). FIG. 89 shows that the value of Vgs101−Vth is almostconstant even when the value of the threshold voltage Vth is changed,and the variation in Vgs101−Vth is less than about 10 to 15%.

FIG. 90 shows the value of Vgs101−Vth obtained by the calculation underCondition B. In FIG. 90 , the horizontal axis represents the value ofthe threshold voltage Vth (V) and the vertical axis represents the valueof Vgs101−Vth (V). In FIG. 90 , the value of Vgs100−Vth is almostconstant when the threshold voltage Vth is positive voltage. Incontrast, when the threshold voltage Vth is negative voltage, the valueof Vgs101−Vth is larger as the threshold voltage Vth of negativepolarity is higher, which means the value of Vgs100−Vth depends on thevalue of the threshold voltage Vth.

The calculation results prove that in the semiconductor device accordingto one aspect of the present invention, the gate-source voltage Vgs101of the transistor 101 can be set by taking the threshold voltage Vth ofthe transistor 11 into consideration, even when the transistor 101 isnormally on, that is, even when the threshold voltage Vth is negativevoltage.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part or all of another embodiment.Thus, part or all of this embodiment can be freely combined with,applied to, or replaced with part or all of another embodiment.

REFERENCE NUMERALS

-   11: switch, 11 t: transistor, 12: switch, 12 t: transistor, 13:    switch, 13 t: transistor, 14: switch, 14 a: switch, 14 b: switch, 14    t: transistor, 15: switch, 21: wiring, 22: wiring, 23: wiring, 23 a:    wiring, 23 b: wiring, 24: wiring, 25: wiring, 26: wiring, 27:    wiring, 31: wiring, 32: wiring, 33: wiring, 34: wiring, 100:    circuit, 101: transistor, 102: capacitor, 103: capacitor, 104: load,    104 a: light-emitting element, 104 b: light-emitting element, 105:    capacitor, 105 a: capacitor, 105 b: capacitor, 201: circuit, 202:    circuit, 203: circuit, 203 a: circuit, 203 b: circuit, 204: circuit,    205: circuit, 206: circuit, 207: circuit, 208: circuit, 220:    circuit, 221: circuit, 222: circuit, 222 a: circuit, 222 b: circuit,    223: circuit, 224: circuit, 225: circuit, 226: circuit, 230:    circuit, 231: circuit, 232: circuit, 233: circuit, 300:    semiconductor film, 301: semiconductor film, 302: conductive film,    303: conductive film, 304: conductive film, 305: conductive film,    306: conductive film, 307: conductive film, 308: conductive film,    309: conductive film, 320: semiconductor film, 321: semiconductor    film, 322: semiconductor film, 323: semiconductor film, 324:    conductive film, 325: conductive film, 326: conductive film, 327:    conductive film, 328: conductive film, 329: conductive film, 330:    conductive film, 331: conductive film, 332: conductive film, 333:    semiconductor film, 501: semiconductor film, 502: insulating film,    503: electrode, 504: conductive film, 505: conductive film, 506:    first region, 507: second region, 508: second region, 509: third    region, 510: third region, 520: insulating film, 520 a: first oxide    insulating film, 520 b: second oxide insulating film, 520 c: third    oxide insulating film, 521: semiconductor film, 522: insulating    film, 523: electrode, 524: conductive film, 525: conductive film,    526: first region, 527: sidewall, 528: insulating film, 529:    opening, 530: insulating film, 530 a: first oxide insulating film,    530 b: second oxide insulating film, 531: semiconductor film, 532:    insulating film, 533: electrode, 534: conductive film, 535:    conductive film, 536: first region, 537: second region, 538: second    region, 539: sidewall, 540: insulating film, 550: second region,    551: second region, 602: gate electrode, 603: gate insulating film,    604: semiconductor film, 605: conductive film, 606: conductive film,    607: insulating film, 612: gate electrode, 613: gate insulating    film, 614: semiconductor film, 615: conductive film, 616: conductive    film, 617: insulating film, 618: channel protective film, 622: gate    electrode, 623: gate insulating film, 624: semiconductor film, 625:    conductive film, 626: conductive film, 627: insulating film, 642:    gate electrode, 643: gate insulating film, 644: semiconductor film,    645: conductive film, 646: conductive film, 647: insulating film,    700: pixel area, 701: driver circuit, 702: driver circuit, 710:    driver circuit, 711: pixel area, 800: substrate, 801: insulating    film, 802: insulating film, 803: insulating film, 814: switch, 914:    switch, 914 t: transistor, 932: wiring, 4001: substrate, 4002: pixel    area, 4003: circuit, 4004: circuit, 4006: substrate, 4007: filler,    4008: transistor, 4009: transistor, 4010: transistor, 4011:    light-emitting element, 4012: counter electrode, 4013:    light-emitting layer, 4014: wiring, 4015: wiring, 4016: connection    terminal, 4017: wiring, 4018: FPC, 4019: anisotropic conductive    film, 4020: sealant, 5001: housing, 5002: housing, 5003: display    portion, 5004: display portion, 5005: microphone, 5006: speaker,    5007: control key, 5008: stylus, 5201: housing, 5202: display    portion, 5203: support, 5401: housing, 5402: display portion, 5403:    keyboard, 5404: pointing device, 5601: housing, 5602: display    portion, 5603: control key, 5801: housing, 5802: display portion,    5803: audio input portion, 5804: audio output portion, 5805: control    key, 5806: light-receiving portion, 5901: housing, 5902: housing,    5903: display portion, 5904: display portion, 5905: joint, 5906:    control key, 9206: circuit, 900501: display panel, 900513: FPC,    900530: housing, 900531: printed board, 900532: speaker, 900533:    microphone, 900534: transmission/reception circuit, 900535: signal    processing circuit, 900536: input unit, 900537: battery, and 900539:    housing.

This application is based on Japanese Patent Application serial No.2011-228418 filed with Japan Patent Office on Oct. 18, 2011 and JapanesePatent Application serial No. 2011-261317 filed with Japan Patent Officeon Nov. 30, 2011, the entire contents of which are hereby incorporatedby reference.

The invention claimed is:
 1. A semiconductor device comprising a pixel,the pixel comprising: first to fourth transistors; and first and secondcapacitors, wherein a gate of the first transistor is electricallyconnected to one of a source and a drain of the third transistor and afirst terminal of the first capacitor, wherein one of a source and adrain of the first transistor is electrically connected to one of asource and a drain of the fourth transistor and a first terminal of thesecond capacitor, wherein the other of the source and the drain of thefourth transistor is electrically connected to one of a source and adrain of the second transistor, a second terminal of the first capacitorand a second terminal of the second capacitor, wherein a semiconductorfilm functions as an active layer of at least one of the first, secondand fourth transistors, and wherein the semiconductor film functions asone of the first terminal of the first capacitor, the second terminal ofthe first capacitor, the first terminal of the second capacitor, and thesecond terminal of the second capacitor.
 2. The semiconductor deviceaccording to claim 1, further comprising: a light-emitting elementelectrically connected to the one of the source and the drain of thefirst transistor.
 3. The semiconductor device according to claim 1,further comprising: a fifth transistor electrically connected to the oneof the source and the drain of the first transistor.
 4. Thesemiconductor device according to claim 1, further comprising: a fifthtransistor electrically connected to the other of the source and thedrain of the first transistor.
 5. The semiconductor device according toclaim 1, wherein the first to fourth transistors are n-channeltransistors.
 6. The semiconductor device according to claim 1, whereinthe semiconductor film comprises an oxide semiconductor.
 7. Asemiconductor device comprising a pixel, the pixel comprising: first tofourth transistors; and first and second capacitors, wherein a gate ofthe first transistor is electrically connected to one of a source and adrain of the third transistor and a first terminal of the firstcapacitor, wherein one of a source and a drain of the first transistoris electrically connected to one of a source and a drain of the fourthtransistor and a first terminal of the second capacitor, wherein theother of the source and the drain of the fourth transistor iselectrically connected to one of a source and a drain of the secondtransistor, and wherein a semiconductor film functions as an activelayer of the second transistor and an active layer of the fourthtransistor.
 8. The semiconductor device according to claim 7, furthercomprising: a light-emitting element electrically connected to the oneof the source and the drain of the first transistor.
 9. A semiconductordevice comprising a pixel, the pixel comprising: a first transistor, asecond transistor, a third transistor, a fourth transistor, and a fifthtransistor; a first capacitor; and a light-emitting element, wherein agate of the first transistor is electrically connected to one of asource and a drain of the third transistor and a first terminal of thefirst capacitor, wherein one of a source and a drain of the firsttransistor is electrically connected to one of a source and a drain ofthe fourth transistor, wherein the other of the source and a drain ofthe first transistor is electrically connected to a first wiring whichis a power supply line, wherein one of a source and a drain of thesecond transistor is electrically connected to a second wiring which isa signal line, wherein one of a source and a drain of the fifthtransistor is electrically connected to the light-emitting element,wherein the other of the source and the drain of the fifth transistor iselectrically connected to a third wiring, wherein a gate of the thirdtransistor is electrically connected to a first gate line, wherein agate of the fourth transistor is electrically connected to a second gateline, wherein a gate of the fifth transistor is electrically connectedto a third gate line, wherein a semiconductor film functions as anactive layer of each of the first transistor, the second transistor, thefourth transistor, and the fifth transistor, and wherein a firstconductive film functioning as the first gate line, a second conductivefilm functioning as the second gate line, and a third conductive filmfunctioning as the third gate line extend in the same direction.
 10. Thesemiconductor device according to claim 9, further comprising: a firstgate driver electrically connected to the first gate line; a second gatedriver electrically connected to the second gate line; and a third gatedriver electrically connected to the third gate line.
 11. Thesemiconductor device according to claim 9, wherein a channel length ofthe first transistor is larger than a channel length of each of thesecond transistor, the third transistor, the fourth transistor, and thefifth transistor.
 12. The semiconductor device according to claim 9,wherein a channel length of the first transistor is greater than 20 μmor more.
 13. The semiconductor device according to claim 9, wherein thesemiconductor film is a polycrystalline semiconductor film.